Hacking Perl Code

[carlriz]

Hey guys, I was reading 'cool uses for perl' and came across some code. What is the '=<<' operator? What is 'eval $begin.$content;' (specifically, you can concat. these?)? Just curious how this works (maybe the rest of the code too).

my $file = $ARGV[0];
my $content = do { open my $fh, '<', $file or die $!; local $/; <$fh> 
+};
my $begin =<<'THEEND';

UNITCHECK {
    no strict; # access $VERSION by symbolic reference
    no warnings qw (uninitialized);
    print map {
        s!/!::!g;
        s!.pm$!!;
        sprintf "%-20s %s\n", $_, ${"${_}::VERSION"}
    } sort keys %INC;
    exit;
};

THEEND

eval $begin.$content;
print $@ if $@;

[kcott]

G'day carlriz,

That's actually an assignment:

my $begin =

With a heredoc:

          <<'THEEND';
    ...
THEEND

For more details, see "perlop: Quote-Like Operators". Scroll down to <<EOF for heredoc specifics. You'll find additional, relevant information in the "Gory details of parsing quoted constructs" section, just after that.

[choroba]

There is no =<< operator. It is an assignment operator = and a HERE-document (see perlop for both).

What's wrong on concatenating two scalar variables, whose contents are strings? Check eval EXPR for the meaning.

[LanX]

You should have better asked here -> "check modules used by a script and their version" or at least link to the original thread.

What it does is to compile a file's content given as parameter within an eval after prepending a UNITCHECK block which is executed right after compilation and exits right away.

Within this block the required (resp used) modules are parsed from the keys of %INC.

But the author is overly optimistic about parsing code without executing it. It was already mentioned that BEGIN blocks are executed at compile time, additionally does every use execute an import function.

And much more magic ...

[Discipulus]

thanks LanX, as suggested I added a link to this post in the original one.

I also added an update to the same post to explain better my optimistic sentence parsing code without executing it. Doing this i discovered that my solution executes even less code than the perl -c -d:TraceUse script.pl one.

HtH
L*

[LanX]

> Doing this i discovered that my solution executes even less code than the perl -c -d:TraceUse script.pl one.

Could you please elaborate what you mean with less code?

Can you provide an example showing the difference?

TraceUse hooks into @INC with a callback like recently discussed.

With this approach its even possible to report before anything is included and to avoid the import.

[Anonymous Monk]

B::Deparse can help you de-obfuscate code a bit

ppi_dumper can help you intrepret code without running it (tells you what friendly manual to read )

$ ppi_dumper carliz-hacking.pl
PPI::Document
  PPI::Token::Whitespace        '\n'
  PPI::Statement::Variable
    PPI::Token::Word    'my'
    PPI::Token::Whitespace      ' '
    PPI::Token::Symbol          '$file'
    PPI::Token::Whitespace      ' '
    PPI::Token::Operator        '='
    PPI::Token::Whitespace      ' '
    PPI::Token::Symbol          '$ARGV'
    PPI::Structure::Subscript   [ ... ]
      PPI::Statement::Expression
        PPI::Token::Number      '0'
    PPI::Token::Structure       ';'
  PPI::Token::Whitespace        '\n'
  PPI::Statement::Variable
    PPI::Token::Word    'my'
    PPI::Token::Whitespace      ' '
    PPI::Token::Symbol          '$content'
    PPI::Token::Whitespace      ' '
    PPI::Token::Operator        '='
    PPI::Token::Whitespace      ' '
    PPI::Token::Word    'do'
    PPI::Token::Whitespace      ' '
    PPI::Structure::Block       { ... }
      PPI::Token::Whitespace    ' '
      PPI::Statement
        PPI::Token::Word        'open'
        PPI::Token::Whitespace          ' '
        PPI::Token::Word        'my'
        PPI::Token::Whitespace          ' '
        PPI::Token::Symbol      '$fh'
        PPI::Token::Operator    ','
        PPI::Token::Whitespace          ' '
        PPI::Token::Quote::Single       ''<''
        PPI::Token::Operator    ','
        PPI::Token::Whitespace          ' '
        PPI::Token::Symbol      '$file'
        PPI::Token::Whitespace          ' '
        PPI::Token::Operator    'or'
        PPI::Token::Whitespace          ' '
        PPI::Token::Word        'die'
        PPI::Token::Whitespace          ' '
        PPI::Token::Magic       '$!'
        PPI::Token::Structure   ';'
      PPI::Token::Whitespace    ' '
      PPI::Statement::Variable
        PPI::Token::Word        'local'
        PPI::Token::Whitespace          ' '
        PPI::Token::Magic       '$/'
        PPI::Token::Structure   ';'
      PPI::Token::Whitespace    ' '
      PPI::Statement
        PPI::Token::Operator    '<'
        PPI::Token::Symbol      '$fh'
        PPI::Token::Operator    '>'
      PPI::Token::Whitespace    ' '
    PPI::Token::Structure       ';'
  PPI::Token::Whitespace        '\n'
  PPI::Statement::Variable
    PPI::Token::Word    'my'
    PPI::Token::Whitespace      ' '
    PPI::Token::Symbol          '$begin'
    PPI::Token::Whitespace      ' '
    PPI::Token::Operator        '='
    PPI::Token::HereDoc         '<<'THEEND''
    PPI::Token::Structure       ';'
  PPI::Token::Whitespace        '\n'
  PPI::Token::Whitespace        '\n'
  PPI::Statement
    PPI::Token::Word    'eval'
    PPI::Token::Whitespace      ' '
    PPI::Token::Symbol          '$begin'
    PPI::Token::Operator        '.'
    PPI::Token::Symbol          '$content'
    PPI::Token::Structure       ';'
  PPI::Token::Whitespace        '\n'
  PPI::Statement
    PPI::Token::Word    'print'
    PPI::Token::Whitespace      ' '
    PPI::Token::Magic   '$@'
    PPI::Token::Whitespace      ' '
    PPI::Token::Word    'if'
    PPI::Token::Whitespace      ' '
    PPI::Token::Magic   '$@'
    PPI::Token::Structure       ';'
  PPI::Token::Whitespace        '\n'
  PPI::Token::Whitespace        '\n'

OPerators are documented in perlop, functions in perlfunc, magic variables in perlvar ... its just a matter of looking up each piece you're unsure about... perldoc -f eval

Tutorials: Perl documentation documentation, Searching Perl Documentation, How to Read The Fine Friendly Manual

[Anonymous Monk]

 PPI::Token::Word    'eval' 

words are function call and things like that

[carlriz]

That's pretty useful. Thanks

[Anonymous Monk]

The my $content = do { ... }; bit is an idiom that reads the entire contents of the file named by $file and stores it in $content.

<< introduces a here doc. Everything between that line and the end marker THEEND is a string that gets stored in $begin.

$begin.$content concatenates the two strings, and eval executes that code.

[carlriz]

Very interesting idiom...I enjoyed the read.