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Perl-Sensitive Sunglasses


by gods
on Aug 25, 1999 at 06:59 UTC ( #413=perlman: print w/ replies, xml ) Need Help??


Current Perl documentation can be found at

Here is our local, out-dated (pre-5.6) version:


perldebug - Perl debugging


First of all, have you tried using the -w switch?

The Perl Debugger

``As soon as we started programming, we found to our surprise that it wasn't as easy to get programs right as we had thought. Debugging had to be discovered. I can remember the exact instant when I realized that a large part of my life from then on was going to be spent in finding mistakes in my own programs.''

  --Maurice Wilkes, 1949

If you invoke Perl with the -d switch, your script runs under the Perl source debugger. This works like an interactive Perl environment, prompting for debugger commands that let you examine source code, set breakpoints, get stack backtraces, change the values of variables, etc. This is so convenient that you often fire up the debugger all by itself just to test out Perl constructs interactively to see what they do. For example:

    perl -d -e 42

In Perl, the debugger is not a separate program as it usually is in the typical compiled environment. Instead, the -d flag tells the compiler to insert source information into the parse trees it's about to hand off to the interpreter. That means your code must first compile correctly for the debugger to work on it. Then when the interpreter starts up, it preloads a Perl library file containing the debugger itself.

The program will halt right before the first run-time executable statement (but see below regarding compile-time statements) and ask you to enter a debugger command. Contrary to popular expectations, whenever the debugger halts and shows you a line of code, it always displays the line it's about to execute, rather than the one it has just executed.

Any command not recognized by the debugger is directly executed (eval'd) as Perl code in the current package. (The debugger uses the DB package for its own state information.)

Leading white space before a command would cause the debugger to think it's NOT a debugger command but for Perl, so be careful not to do that.

Debugger Commands

The debugger understands the following commands:

h [command]

Prints out a help message.

If you supply another debugger command as an argument to the h command, it prints out the description for just that command. The special argument of h h produces a more compact help listing, designed to fit together on one screen.

If the output of the h command (or any command, for that matter) scrolls past your screen, either precede the command with a leading pipe symbol so it's run through your pager, as in

    DB> |h

You may change the pager which is used via O pager=... command.

p expr

Same as print {$DB::OUT} expr in the current package. In particular, because this is just Perl's own print function, this means that nested data structures and objects are not dumped, unlike with the x command.

The DB::OUT filehandle is opened to /dev/tty, regardless of where STDOUT may be redirected to.

x expr

Evaluates its expression in list context and dumps out the result in a pretty-printed fashion. Nested data structures are printed out recursively, unlike the print function.

The details of printout are governed by multiple Options.

V [pkg [vars]]

Display all (or some) variables in package (defaulting to the main package) using a data pretty-printer (hashes show their keys and values so you see what's what, control characters are made printable, etc.). Make sure you don't put the type specifier (like $) there, just the symbol names, like this:

    V DB filename line

Use ~pattern and !pattern for positive and negative regexps.

Nested data structures are printed out in a legible fashion, unlike the print function.

The details of printout are governed by multiple Options.

X [vars]

Same as V currentpackage [vars].


Produce a stack backtrace. See below for details on its output.

s [expr]

Single step. Executes until it reaches the beginning of another statement, descending into subroutine calls. If an expression is supplied that includes function calls, it too will be single-stepped.

n [expr]

Next. Executes over subroutine calls, until it reaches the beginning of the next statement. If an expression is supplied that includes function calls, those functions will be executed with stops before each statement.


Repeat last n or perlman:perlop command.

c [line|sub]

Continue, optionally inserting a one-time-only breakpoint at the specified line or subroutine.


List next window of lines.

l min+incr

List incr+1 lines starting at min.

l min-max

List lines min through max. l - is synonymous to -.

l line

List a single line.

l subname

List first window of lines from subroutine.


List previous window of lines.

w [line]

List window (a few lines) around the current line.


Return debugger pointer to the last-executed line and print it out.

f filename

Switch to viewing a different file or eval statement. If filename is not a full filename as found in values of %INC, it is considered as a regexp.


Search forwards for pattern; final / is optional.


Search backwards for pattern; final ? is optional.


List all breakpoints and actions.

S [[!]pattern]

List subroutine names [not] matching pattern.


Toggle trace mode (see also AutoTrace Option).

t expr

Trace through execution of expr. For example:

 $ perl -de 42
 Stack dump during die enabled outside of evals.

 Loading DB routines from patch level 0.94
 Emacs support available.

 Enter h or `h h' for help.

 main::(-e:1):   0
   DB<1> sub foo { 14 }

   DB<2> sub bar { 3 }

   DB<3> t print foo() * bar()
 main::((eval 172):3):   print foo() + bar();
 main::foo((eval 168):2):
 main::bar((eval 170):2):

or, with the Option frame=2 set,

   DB<4> O f=2
                frame = '2'
   DB<5> t print foo() * bar()
 3:      foo() * bar()
 entering main::foo
  2:     sub foo { 14 };
 exited main::foo
 entering main::bar
  2:     sub bar { 3 };
 exited main::bar
b [line] [condition]

Set a breakpoint. If line is omitted, sets a breakpoint on the line that is about to be executed. If a condition is specified, it's evaluated each time the statement is reached and a breakpoint is taken only if the condition is true. Breakpoints may be set on only lines that begin an executable statement. Conditions don't use if:

    b 237 $x > 30
    b 237 ++$count237 < 11
    b 33 /pattern/i
b subname [condition]

Set a breakpoint at the first line of the named subroutine.

b postpone subname [condition]

Set breakpoint at first line of subroutine after it is compiled.

b load filename

Set breakpoint at the first executed line of the file. Filename should be a full name as found in values of %INC.

b compile subname

Sets breakpoint at the first statement executed after the subroutine is compiled.

d [line]

Delete a breakpoint at the specified line. If line is omitted, deletes the breakpoint on the line that is about to be executed.


Delete all installed breakpoints.

a [line] command

Set an action to be done before the line is executed. The sequence of steps taken by the debugger is

  1. check for a breakpoint at this line
  2. print the line if necessary (tracing)
  3. do any actions associated with that line
  4. prompt user if at a breakpoint or in single-step
  5. evaluate line

For example, this will print out $foo every time line 53 is passed:

    a 53 print "DB FOUND $foo\n"

Delete all installed actions.

W [expr]

Add a global watch-expression.


Delete all watch-expressions.

O [opt[=val]] [opt"val"] [opt?]...

Set or query values of options. val defaults to 1. opt can be abbreviated. Several options can be listed.
recallCommand, ShellBang

The characters used to recall command or spawn shell. By default, these are both set to !.


Program to use for output of pager-piped commands (those beginning with a | character.) By default, $ENV{PAGER} will be used.


Run Tk while prompting (with ReadLine).

signalLevel, warnLevel, dieLevel

Level of verbosity. By default the debugger is in a sane verbose mode, thus it will print backtraces on all the warnings and die-messages which are going to be printed out, and will print a message when interesting uncaught signals arrive.

To disable this behaviour, set these values to 0. If dieLevel is 2, then the messages which will be caught by surrounding eval are also printed.


Trace mode (similar to t command, but can be put into PERLDB_OPTS).


File or pipe to print line number info to. If it is a pipe (say, |visual_perl_db), then a short, ``emacs like'' message is used.


If 0, allows stepping off the end of the script.


affects printing of return value after r command.


affects screen appearance of the command line (see ReadLine).


affects printing messages on entry and exit from subroutines. If frame & 2 is false, messages are printed on entry only. (Printing on exit may be useful if inter(di)spersed with other messages.)

If frame & 4, arguments to functions are printed as well as the context and caller info. If frame & 8, overloaded stringify and tied FETCH are enabled on the printed arguments. If frame & 16, the return value from the subroutine is printed as well.

The length at which the argument list is truncated is governed by the next option:


length at which the argument list is truncated when frame option's bit 4 is set.

The following options affect what happens with V, X, and x commands:

arrayDepth, hashDepth

Print only first N elements ('' for all).

compactDump, veryCompact

Change style of array and hash dump. If compactDump, short array may be printed on one line.


Whether to print contents of globs.


Dump arrays holding debugged files.


Dump symbol tables of packages.


Dump contents of ``reused'' addresses.

quote, HighBit, undefPrint

Change style of string dump. Default value of quote is auto, one can enable either double-quotish dump, or single-quotish by setting it to " or '. By default, characters with high bit set are printed as is.


very rudimentally per-package memory usage dump. Calculates total size of strings in variables in the package.

During startup options are initialized from $ENV{PERLDB_OPTS}. You can put additional initialization options TTY, noTTY, ReadLine, and NonStop there.

Example rc file:

  &parse_options("NonStop=1 LineInfo=db.out AutoTrace");

The script will run without human intervention, putting trace information into the file db.out. (If you interrupt it, you would better reset LineInfo to something ``interactive''!)


The TTY to use for debugging I/O.


If set, goes in NonStop mode, and would not connect to a TTY. If interrupt (or if control goes to debugger via explicit setting of $DB::signal or $DB::single from the Perl script), connects to a TTY specified by the TTY option at startup, or to a TTY found at runtime using Term::Rendezvous module of your choice.

This module should implement a method new which returns an object with two methods: IN and OUT, returning two filehandles to use for debugging input and output correspondingly. Method new may inspect an argument which is a value of $ENV{PERLDB_NOTTY} at startup, or is "/tmp/perldbtty$$" otherwise.


If false, readline support in debugger is disabled, so you can debug ReadLine applications.


If set, debugger goes into noninteractive mode until interrupted, or programmatically by setting $DB::signal or $DB::single.

Here's an example of using the $ENV{PERLDB_OPTS} variable:

  $ PERLDB_OPTS="N f=2" perl -d myprogram

will run the script myprogram without human intervention, printing out the call tree with entry and exit points. Note that N f=2 is equivalent to NonStop=1 frame=2. Note also that at the moment when this documentation was written all the options to the debugger could be uniquely abbreviated by the first letter (with exception of Dump* options).

Other examples may include

  $ PERLDB_OPTS="N f A L=listing" perl -d myprogram

- runs script noninteractively, printing info on each entry into a subroutine and each executed line into the file listing. (If you interrupt it, you would better reset LineInfo to something ``interactive''!)

  $ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogram

may be useful for debugging a program which uses Term::ReadLine itself. Do not forget detach shell from the TTY in the window which corresponds to /dev/ttyc, say, by issuing a command like

  $ sleep 1000000

See Debugger Internals below for more details.

lt [ command ]

Set an action (Perl command) to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines. If command is missing, resets the list of actions.

ltlt command

Add an action (Perl command) to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines.

gt command

Set an action (Perl command) to happen after the prompt when you've just given a command to return to executing the script. A multi-line command may be entered by backslashing the newlines. If command is missing, resets the list of actions.

gtgt command

Adds an action (Perl command) to happen after the prompt when you've just given a command to return to executing the script. A multi-line command may be entered by backslashing the newlines.

{ [ command ]

Set an action (debugger command) to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines. If command is missing, resets the list of actions.

{{ command

Add an action (debugger command) to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines.

! number

Redo a previous command (default previous command).

! -number

Redo number'th-to-last command.

! pattern

Redo last command that started with pattern. See O recallCommand, too.

!! cmd

Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See O shellBang too.

H -number

Display last n commands. Only commands longer than one character are listed. If number is omitted, lists them all.

q or ^D

Quit. (``quit'' doesn't work for this.) This is the only supported way to exit the debugger, though typing exit twice may do it too.

Set an Option inhibit_exit to 0 if you want to be able to step off the end the script. You may also need to set $finished to 0 at some moment if you want to step through global destruction.


Restart the debugger by execing a new session. It tries to maintain your history across this, but internal settings and command line options may be lost.

Currently the following setting are preserved: history, breakpoints, actions, debugger Options, and the following command line options: -w, -I, and -e.


Run debugger command, piping DB::OUT to current pager.


Same as |dbcmd but DB::OUT is temporarily selected as well. Often used with commands that would otherwise produce long output, such as

    |V main
= [alias value]

Define a command alias, like

    = quit q

or list current aliases.


Execute command as a Perl statement. A missing semicolon will be supplied.

m expr

The expression is evaluated, and the methods which may be applied to the result are listed.

m package

The methods which may be applied to objects in the package are listed.

Debugger input/output


The debugger prompt is something like


or even


where that number is the command number, which you'd use to access with the builtin csh-like history mechanism, e.g., !17 would repeat command number 17. The number of angle brackets indicates the depth of the debugger. You could get more than one set of brackets, for example, if you'd already at a breakpoint and then printed out the result of a function call that itself also has a breakpoint, or you step into an expression via s/n/t expression command.

Multiline commands

If you want to enter a multi-line command, such as a subroutine definition with several statements, or a format, you may escape the newline that would normally end the debugger command with a backslash. Here's an example:

      DB<1> for (1..4) {         \
      cont:     print "ok\n";   \
      cont: }

Note that this business of escaping a newline is specific to interactive commands typed into the debugger.

Stack backtrace

Here's an example of what a stack backtrace via T command might look like:

    $ = main::infested called from file `' line 10
    @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
    $ = main::pests('bactrian', 4) called from file `camel_flea' line 4

The left-hand character up there tells whether the function was called in a scalar or list context (we bet you can tell which is which). What that says is that you were in the function main::infested when you ran the stack dump, and that it was called in a scalar context from line 10 of the file, but without any arguments at all, meaning it was called as &infested. The next stack frame shows that the function Ambulation::legs was called in a list context from the camel_flea file with four arguments. The last stack frame shows that main::pests was called in a scalar context, also from camel_flea, but from line 4.

Note that if you execute T command from inside an active use statement, the backtrace will contain both require frame and an eval) frame.


Listing given via different flavors of l command looks like this:

    DB<<13>> l
  101:                @i{@i} = ();
  102:b               @isa{@i,$pack} = ()
  103                     if(exists $i{$prevpack} || exists $isa{$pack});
  104             }
  106             next
  107==>              if(exists $isa{$pack});
  109:a           if ($extra-- > 0) {
  110:                %isa = ($pack,1);

Note that the breakable lines are marked with :, lines with breakpoints are marked by b, with actions by a, and the next executed line is marked by ==>.

Frame listing

When frame option is set, debugger would print entered (and optionally exited) subroutines in different styles.

What follows is the start of the listing of

  env "PERLDB_OPTS=f=n N" perl -d -V

for different values of n:

  1.   entering main::BEGIN
       entering Config::BEGIN
        Package lib/
        Package lib/
       Package lib/
       entering Config::TIEHASH
       entering Exporter::import
        entering Exporter::export
      entering Config::myconfig
       entering Config::FETCH
       entering Config::FETCH
       entering Config::FETCH
       entering Config::FETCH
  2.   entering main::BEGIN
       entering Config::BEGIN
        Package lib/
        Package lib/
       exited Config::BEGIN
       Package lib/
       entering Config::TIEHASH
       exited Config::TIEHASH
       entering Exporter::import
        entering Exporter::export
        exited Exporter::export
       exited Exporter::import
      exited main::BEGIN
      entering Config::myconfig
       entering Config::FETCH
       exited Config::FETCH
       entering Config::FETCH
       exited Config::FETCH
       entering Config::FETCH
  3.   in  $=main::BEGIN() from /dev/nul:0
       in  $=Config::BEGIN() from lib/
        Package lib/
        Package lib/
       Package lib/
       in  $=Config::TIEHASH('Config') from lib/
       in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
        in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
      in  @=Config::myconfig() from /dev/nul:0
       in  $=Config::FETCH(ref(Config), 'package') from lib/
       in  $=Config::FETCH(ref(Config), 'baserev') from lib/
       in  $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/
       in  $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/
       in  $=Config::FETCH(ref(Config), 'osname') from lib/
       in  $=Config::FETCH(ref(Config), 'osvers') from lib/
  4.   in  $=main::BEGIN() from /dev/nul:0
       in  $=Config::BEGIN() from lib/
        Package lib/
        Package lib/
       out $=Config::BEGIN() from lib/
       Package lib/
       in  $=Config::TIEHASH('Config') from lib/
       out $=Config::TIEHASH('Config') from lib/
       in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
        in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
        out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
       out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
      out $=main::BEGIN() from /dev/nul:0
      in  @=Config::myconfig() from /dev/nul:0
       in  $=Config::FETCH(ref(Config), 'package') from lib/
       out $=Config::FETCH(ref(Config), 'package') from lib/
       in  $=Config::FETCH(ref(Config), 'baserev') from lib/
       out $=Config::FETCH(ref(Config), 'baserev') from lib/
       in  $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/
       out $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/
       in  $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/
  5. 4

      in  $=main::BEGIN() from /dev/nul:0
       in  $=Config::BEGIN() from lib/
        Package lib/
        Package lib/
       out $=Config::BEGIN() from lib/
       Package lib/
       in  $=Config::TIEHASH('Config') from lib/
       out $=Config::TIEHASH('Config') from lib/
       in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
        in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
        out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
       out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
      out $=main::BEGIN() from /dev/nul:0
      in  @=Config::myconfig() from /dev/nul:0
       in  $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/
       out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/
       in  $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/
       out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/
  6. 0

      in  $=CODE(0x15eca4)() from /dev/null:0
       in  $=CODE(0x182528)() from lib/
        Package lib/
       out $=CODE(0x182528)() from lib/
       scalar context return from CODE(0x182528): undef
       Package lib/
       in  $=Config::TIEHASH('Config') from lib/
       out $=Config::TIEHASH('Config') from lib/
       scalar context return from Config::TIEHASH:   empty hash
       in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
        in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
        out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
        scalar context return from Exporter::export: ''
       out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
       scalar context return from Exporter::import: ''

In all the cases indentation of lines shows the call tree, if bit 2 of frame is set, then a line is printed on exit from a subroutine as well, if bit 4 is set, then the arguments are printed as well as the caller info, if bit 8 is set, the arguments are printed even if they are tied or references, if bit 16 is set, the return value is printed as well.

When a package is compiled, a line like this

    Package lib/

is printed with proper indentation.

Debugging compile-time statements

If you have any compile-time executable statements (code within a BEGIN block or a use statement), these will NOT be stopped by debugger, although requires will (and compile-time statements can be traced with AutoTrace option set in PERLDB_OPTS). From your own Perl code, however, you can transfer control back to the debugger using the following statement, which is harmless if the debugger is not running:

    $DB::single = 1;

If you set $DB::single to the value 2, it's equivalent to having just typed the n command, whereas a value of 1 means the perlman:perlop command. The $DB::trace variable should be set to 1 to simulate having typed the t command.

Another way to debug compile-time code is to start debugger, set a breakpoint on load of some module thusly

    DB<7> b load f:/perllib/lib/
  Will stop on load of `f:/perllib/lib/'.

and restart debugger by R command (if possible). One can use b compile subname for the same purpose.

Debugger Customization

Most probably you do not want to modify the debugger, it contains enough hooks to satisfy most needs. You may change the behaviour of debugger from the debugger itself, using Options, from the command line via PERLDB_OPTS environment variable, and from customization files.

You can do some customization by setting up a .perldb file which contains initialization code. For instance, you could make aliases like these (the last one is one people expect to be there):

    $DB::alias{'len'}  = 's/^len(.*)/p length($1)/';
    $DB::alias{'stop'} = 's/^stop (at|in)/b/';
    $DB::alias{'ps'}   = 's/^ps\b/p scalar /';
    $DB::alias{'quit'} = 's/^quit(\s*)/exit\$/';

One changes options from .perldb file via calls like this one;

    parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");

(the code is executed in the package DB). Note that .perldb is processed before processing PERLDB_OPTS. If .perldb defines the subroutine afterinit, it is called after all the debugger initialization ends. .perldb may be contained in the current directory, or in the perlman:perlrun/perlman:perlrun directory.

If you want to modify the debugger, copy from the Perl library to another name and modify it as necessary. You'll also want to set your perlman:perlrun environment variable to say something like this:

    BEGIN { require "" }

As the last resort, one can use perlman:perlrun to customize debugger by directly setting internal variables or calling debugger functions.

Readline Support

As shipped, the only command line history supplied is a simplistic one that checks for leading exclamation points. However, if you install the Term::ReadKey and Term::ReadLine modules from CPAN, you will have full editing capabilities much like GNU readline(3) provides. Look for these in the modules/by-module/Term directory on CPAN.

A rudimentary command line completion is also available. Unfortunately, the names of lexical variables are not available for completion.

Editor Support for Debugging

If you have GNU emacs installed on your system, it can interact with the Perl debugger to provide an integrated software development environment reminiscent of its interactions with C debuggers.

Perl is also delivered with a start file for making emacs act like a syntax-directed editor that understands (some of) Perl's syntax. Look in the emacs directory of the Perl source distribution.

(Historically, a similar setup for interacting with vi and the X11 window system had also been available, but at the time of this writing, no debugger support for vi currently exists.)

The Perl Profiler

If you wish to supply an alternative debugger for Perl to run, just invoke your script with a colon and a package argument given to the -d flag. One of the most popular alternative debuggers for Perl is DProf, the Perl profiler. As of this writing, DProf is not included with the standard Perl distribution, but it is expected to be included soon, for certain values of ``soon''.

Meanwhile, you can fetch the Devel::Dprof module from CPAN. Assuming it's properly installed on your system, to profile your Perl program in the file, just type:

    perl -d:DProf

When the script terminates the profiler will dump the profile information to a file called tmon.out. A tool like dprofpp (also supplied with the Devel::DProf package) can be used to interpret the information which is in that profile.

Debugger support in perl

When you call the caller function (see caller) from the package DB, Perl sets the array @DB::args to contain the arguments the corresponding stack frame was called with.

If perl is run with -d option, the following additional features are enabled (cf. perlman:perlvar):

  • Perl inserts the contents of $ENV{PERL5DB} (or BEGIN {require ''} if not present) before the first line of the application.

  • The array @{"_<$filename"} is the line-by-line contents of $filename for all the compiled files. Same for evaled strings which contain subroutines, or which are currently executed. The $filename for evaled strings looks like (eval 34).

  • The hash %{"_<$filename"} contains breakpoints and action (it is keyed by line number), and individual entries are settable (as opposed to the whole hash). Only true/false is important to Perl, though the values used by have the form "$break_condition\0$action". Values are magical in numeric context: they are zeros if the line is not breakable.

    Same for evaluated strings which contain subroutines, or which are currently executed. The $filename for evaled strings looks like (eval 34).

  • The scalar ${"_<$filename"} contains "_<$filename". Same for evaluated strings which contain subroutines, or which are currently executed. The $filename for evaled strings looks like (eval 34).

  • After each required file is compiled, but before it is executed, DB::postponed(*{"_<$filename"}) is called (if subroutine DB::postponed exists). Here the $filename is the expanded name of the required file (as found in values of %INC).

  • After each subroutine subname is compiled existence of $DB::postponed{subname} is checked. If this key exists, DB::postponed(subname) is called (if subroutine DB::postponed exists).

  • A hash %DB::sub is maintained, with keys being subroutine names, values having the form filename:startline-endline. filename has the form (eval 31) for subroutines defined inside evals.

  • When execution of the application reaches a place that can have a breakpoint, a call to DB::DB() is performed if any one of variables $DB::trace, $DB::single, or $DB::signal is true. (Note that these variables are not localizable.) This feature is disabled when the control is inside DB::DB() or functions called from it (unless $^D & (1<<30)).

  • When execution of the application reaches a subroutine call, a call to &DB::sub(args) is performed instead, with $DB::sub being the name of the called subroutine. (Unless the subroutine is compiled in the package DB.)

Note that if &DB::sub needs some external data to be setup for it to work, no subroutine call is possible until this is done. For the standard debugger $DB::deep (how many levels of recursion deep into the debugger you can go before a mandatory break) gives an example of such a dependency.

The minimal working debugger consists of one line

  sub DB::DB {}

which is quite handy as contents of perlman:perlrun environment variable:

  env "PERL5DB=sub DB::DB {}" perl -d your-script

Another (a little bit more useful) minimal debugger can be created with the only line being

  sub DB::DB {print ++$i; scalar <STDIN>}

This debugger would print the sequential number of encountered statement, and would wait for your CR to continue.

The following debugger is quite functional:

    package DB;
    sub DB  {}
    sub sub {print ++$i, " $sub\n"; &$sub}

It prints the sequential number of subroutine call and the name of the called subroutine. Note that &DB::sub should be compiled into the package DB.

Debugger Internals

At the start, the debugger reads your rc file (./.perldb or ~/.perldb under Unix), which can set important options. This file may define a subroutine &afterinit to be executed after the debugger is initialized.

After the rc file is read, the debugger reads environment variable PERLDB_OPTS and parses it as a rest of O ... line in debugger prompt.

It also maintains magical internal variables, such as @DB::dbline, %DB::dbline, which are aliases for @{"::_<current_file"} %{"::_<current_file"}. Here current_file is the currently selected (with the debugger's f command, or by flow of execution) file.

Some functions are provided to simplify customization. See Debugger Customization for description of DB::parse_options(string). The function DB::dump_trace(skip[, count]) skips the specified number of frames, and returns a list containing info about the caller frames (all if count is missing). Each entry is a hash with keys context ($ or @), sub (subroutine name, or info about eval), args (undef or a reference to an array), file, and line.

The function DB::print_trace(FH, skip[, count[, short]]) prints formatted info about caller frames. The last two functions may be convenient as arguments to <, << commands.

Other resources

You did try the -w switch, didn't you?


You cannot get the stack frame information or otherwise debug functions that were not compiled by Perl, such as C or C++ extensions.

If you alter your @_ arguments in a subroutine (such as with shift or pop, the stack backtrace will not show the original values.

Debugging Perl memory usage

Perl is very frivolous with memory. There is a saying that to estimate memory usage of Perl, assume a reasonable algorithm of allocation, and multiply your estimages by 10. This is not absolutely true, but may give you a good grasp of what happens.

Say, an integer cannot take less than 20 bytes of memory, a float cannot take less than 24 bytes, a string cannot take less than 32 bytes (all these examples assume 32-bit architectures, the result are much worse on 64-bit architectures). If a variable is accessed in two of three different ways (which require an integer, a float, or a string), the memory footprint may increase by another 20 bytes. A sloppy malloc() implementation will make these numbers yet more.

On the opposite end of the scale, a declaration like

  sub foo;

may take (on some versions of perl) up to 500 bytes of memory.

Off-the-cuff anecdotal estimates of a code bloat give a factor around 8. This means that the compiled form of reasonable (commented indented etc.) code will take approximately 8 times more than the disk space the code takes.

There are two Perl-specific ways to analyze the memory usage: $ENV{PERL_DEBUG_MSTATS} and -DL switch. First one is available only if perl is compiled with Perl's malloc(), the second one only if Perl compiled with -DDEBUGGING (as with giving -D optimise=-g option to Configure).


If your perl is using Perl's malloc(), and compiled with correct switches (this is the default), then it will print memory usage statistics after compiling your code (if $ENV{PERL_DEBUG_MSTATS} > 1), and before termination of the script (if $ENV{PERL_DEBUG_MSTATS} >= 1). The report format is similar to one in the following example:

  env PERL_DEBUG_MSTATS=2 perl -e "require Carp"
  Memory allocation statistics after compilation: (buckets 4(4)..8188(8192)
     14216 free:   130   117    28     7     9   0   2     2   1 0 0
                437    61    36     0     5
     60924 used:   125   137   161    55     7   8   6    16   2 0 1
                 74   109   304    84    20
  Total sbrk(): 77824/21:119. Odd ends: pad+heads+chain+tail: 0+636+0+2048.
  Memory allocation statistics after execution:   (buckets 4(4)..8188(8192)
     30888 free:   245    78    85    13     6   2   1     3   2 0 1
                315   162    39    42    11
    175816 used:   265   176  1112   111    26  22  11    27   2 1 1
                196   178  1066   798    39
  Total sbrk(): 215040/47:145. Odd ends: pad+heads+chain+tail: 0+2192+0+6144.

It is possible to ask for such a statistic at arbitrary moment by usind Devel::Peek::mstats() (module Devel::Peek is available on CPAN).

Here is the explanation of different parts of the format:


Perl's malloc() uses bucketed allocations. Every request is rounded up to the closest bucket size available, and a bucket of these size is taken from the pool of the buckets of this size.

The above line describes limits of buckets currently in use. Each bucket has two sizes: memory footprint, and the maximal size of user data which may be put into this bucket. Say, in the above example the smallest bucket is both sizes 4. The biggest bucket has usable size 8188, and the memory footprint 8192.

With debugging Perl some buckets may have negative usable size. This means that these buckets cannot (and will not) be used. For greater buckets the memory footprint may be one page greater than a power of 2. In such a case the corresponding power of two is printed instead in the APPROX field above.


The following 1 or 2 rows of numbers correspond to the number of buckets of each size between SMALLEST and GREATEST. In the first row the sizes (memory footprints) of buckets are powers of two (or possibly one page greater). In the second row (if present) the memory footprints of the buckets are between memory footprints of two buckets ``above''.

Say, with the above example the memory footprints are (with current algorith)

     free:    8     16    32    64    128  256 512 1024 2048 4096 8192
           4     12    24    48    80

With non-DEBUGGING perl the buckets starting from 128-long ones have 4-byte overhead, thus 8192-long bucket may take up to 8188-byte-long allocations.


The first two fields give the total amount of memory perl sbrk()ed, and number of sbrk()s used. The third number is what perl thinks about continuity of returned chunks. As far as this number is positive, malloc() will assume that it is probable that sbrk() will provide continuous memory.

The amounts sbrk()ed by external libraries is not counted.

pad: 0

The amount of sbrk()ed memory needed to keep buckets aligned.

heads: 2192

While memory overhead of bigger buckets is kept inside the bucket, for smaller buckets it is kept in separate areas. This field gives the total size of these areas.

chain: 0

malloc() may want to subdivide a bigger bucket into smaller buckets. If only a part of the deceased-bucket is left non-subdivided, the rest is kept as an element of a linked list. This field gives the total size of these chunks.

tail: 6144

To minimize amount of sbrk()s malloc() asks for more memory. This field gives the size of the yet-unused part, which is sbrk()ed, but never touched.

Example of using -DL switch

Below we show how to analyse memory usage by

  do 'lib/auto/POSIX/autosplit.ix';

The file in question contains a header and 146 lines similar to

  sub getcwd ;

Note: the discussion below supposes 32-bit architecture. In the newer versions of perl the memory usage of the constructs discussed here is much improved, but the story discussed below is a real-life story. This story is very terse, and assumes more than cursory knowledge of Perl internals.

Here is the itemized list of Perl allocations performed during parsing of this file:

 !!! "after" at line 3.
    Id  subtot   4   8  12  16  20  24  28  32  36  40  48  56  64  72  80 80+
  0 02   13752   .   .   .   . 294   .   .   .   .   .   .   .   .   .   .   4
  0 54    5545   .   .   8 124  16   .   .   .   1   1   .   .   .   .   .   3
  5 05      32   .   .   .   .   .   .   .   1   .   .   .   .   .   .   .   .
  6 02    7152   .   .   .   .   .   .   .   .   .   . 149   .   .   .   .   .
  7 02    3600   .   .   .   .   . 150   .   .   .   .   .   .   .   .   .   .
  7 03      64   .  -1   .   1   .   .   2   .   .   .   .   .   .   .   .   .
  7 04    7056   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   7
  7 17   38404   .   .   .   .   .   .   .   1   .   . 442 149   .   . 147   .
  9 03    2078  17 249  32   .   .   .   .   2   .   .   .   .   .   .   .   .

To see this list insert two perlman:perlguts statements around the call:

  do 'lib/auto/POSIX/autosplit.ix';
  warn('!!! "after"');

and run it with -DL option. The first warn() will print memory allocation info before the parsing of the file, and will memorize the statistics at this point (we ignore what it prints). The second warn() will print increments w.r.t. this memorized statistics. This is the above printout.

Different Ids on the left correspond to different subsystems of perl interpreter, they are just first argument given to perl memory allocation API New(). To find what 9 03 means grep the perl source for 903. You will see that it is util.c, function savepvn(). This function is used to store a copy of existing chunk of memory. Using C debugger, one can see that it is called either directly from gv_init(), or via sv_magic(), and gv_init() is called from gv_fetchpv() - which is called from newSUB().

Note: to reach this place in debugger and skip all the calls to savepvn during the compilation of the main script, set a C breakpoint in Perl_warn(), continue this point is reached, then set breakpoint in Perl_savepvn(). Note that you may need to skip a handful of Perl_savepvn() which do not correspond to mass production of CVs (there are more 903 allocations than 146 similar lines of lib/auto/POSIX/autosplit.ix). Note also that Perl_ prefixes are added by macroization code in perl header files to avoid conflicts with external libraries.

Anyway, we see that 903 ids correspond to creation of globs, twice per glob - for glob name, and glob stringification magic.

Here are explanations for other Ids above:

  1. 7

    is for creation of bigger XPV* structures. In the above case it creates 3 perlman:perlguts per subroutine, one for a list of lexical variable names, one for a scratchpad (which contains lexical variables and targets), and one for the array of scratchpads needed for recursion.

    It also creates a GV and a CV per subroutine (all called from start_subparse()).

  2. 2 Creates C array corresponding to the perlman:perlguts of scratchpads, and the scratchpad itself (the first fake entry of this scratchpad is created though the subroutine itself is not defined yet).

    It also creates C arrays to keep data for the stash (this is one HV, but it grows, thus there are 4 big allocations: the big chunks are not freeed, but are kept as additional arenas for perlman:perlguts allocations).

  3. 4 creates a HEK for the name of the glob for the subroutine (this name is a key in a stash).

    Big allocations with this Id correspond to allocations of new arenas to keep HE.

  4. 2 creates a GP for the glob for the subroutine.

  5. 2 creates the MAGIC for the glob for the subroutine.

  6. 4 creates arenas which keep SVs.

-DL details

If Perl is run with -DL option, then warn()s which start with `!' behave specially. They print a list of categories of memory allocations, and statistics of allocations of different sizes for these categories.

If warn() string starts with


print changed categories only, print the differences in counts of allocations;


print grown categories only; print the absolute values of counts, and totals;


print nonempty categories, print the absolute values of counts and totals.

Limitations of -DL statistic

If an extension or an external library does not use Perl API to allocate memory, these allocations are not counted.

Debugging regular expressions

There are two ways to enable debugging output for regular expressions.

If your perl is compiled with -DDEBUGGING, you may use the -Dr flag on the command line.

Otherwise, one can use re 'debug', which has effects both at compile time, and at run time (and is not lexically scoped).


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