An interesting discovery about local has emerged in the course of debugging Class::MakeMethods (thanks to an excellent bug report from Adam Spiers).

Take a moment to predict the output of the code below before you run it. 

#!/usr/bin/perl -w
use strict;

print_args( 123, 456, 789 );

sub print_args {
  tamper_args( 'All your arguments are belong to us.' );
  print "My arguments were: " .  join(', ', @_) . "\n";
}

sub tamper_args {
  local @_ = ( 'This is not a potato.' );
}

[download]

On my machine (perl v5.6.1 for darwin), the call to tamper_args somehow clobbers the argument list visible to print_args, producing the following startling result: "My arguments were: All your arguments are belong to us."

This appears to be a conflict between the restoration of local values at the end of a sub {...} scope and the restoration of @_ that always happens at the end of a sub. In particular, the problem goes away if the local is inside another { } block (presumably because the two scope-exits are handled separately). It also goes away if tamper_args is called without a parenthesized argument list (presumably because there's no implicit local-ization in that case.

Now, I'll grant that there's generally no need to local @_ like this, but I've also not been able to find anything that says you're not allowed to do so -- perhaps this consitutes an actual Perl bug?

In reply to Don't local(@_) by simonm

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