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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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    [Corion]: ambrus: AnyEvent(::HTTP) doesn't integrate well with Prima, that's my main problem
    [Corion]: There is a weirdo shim because there is a POE integration for Prima, and if you use that, you can use the POE adapter of AnyEvent. What I'd want is something transport agnostic that parses HTTP or produces HTTP output, so that the communication with ...
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