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I've just spent more than one hour debugging very wierd problem. Test script for one of my projects I'm currently working on died with error like:
Modification of a read-only value attempted at /usr/share/perl5/Net/DN +S/ line 235. Compilation failed in require at /usr/share/perl5/Net/ line 23. BEGIN failed--compilation aborted at /usr/share/perl5/Net/ line +23. Compilation failed in require at /usr/share/perl5/Mail/ li +ne 28. BEGIN failed--compilation aborted at /usr/share/perl5/Mail/CheckUser.p +m line 30. .... ....
Let's look in Net/DNS/
.... sub res_init { if ($os eq "unix") { res_init_unix(); } .... } .... sub res_init_unix { read_config($resolv_conf) if (-f $resolv_conf) and (-r $resolv +_conf); .... } .... sub read_config { .... while (<FILE>) { <------ This is line 235 where it fails .... } .... res_init()
As you can see on load of Net::DNS::Resolver module it tries to read DNS resolver configuration file in subroutine read_config() if it runs on Unix and there exist this file. Nothing wrong with it at first look. Yet it fails with 'Modification of a read-only value attempted' error on while (<FILE>) { line.

Turned out that I had some code which could load Net::DNS module on demand and this code was called inside of map. My next step was finding that following snipplet dies with same error. map { require Net::DNS } 1;
Than I removed all non-essential details and I got final test case: map { while(<>) { } } 1;
Suddendly this error started to make sense to me. map associates $_ to elements of the passed list. In this case it is a list with a single element - constant 1. As it is a constant $_ becomes read-only but while(<>) { } still tries to assign data obtained from file handle to $_ and it results in this error.

So what is the conclusion? Do not use while(<>) { } unless you localize $_ or you are completly sure that nobody will call you code from map (or grep, or for/foreach - they are subject to the same problem). Personally I prefer always using while(my $line = <>) construct.

Update: These nodes demonstrate same problem with while(<>) { ... }: Method call to tied hash leads to file read error and opening a file destroys nulling entries in a list?!?!.

Ilya Martynov (

In reply to while(<>) { ... } considered harmful by IlyaM

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