The following code is a simple hello world program preceded with a source filter that doesn't actually change the code in any way. This code works for me (in either perl 5.10.0 or 5.8.8 on x86-linux), that is, it prints hello world.

use Filter::Util::Call;
BEGIN{ filter_add(sub{
        my $s = filter_read;
        my $t = $_;
        $_ = $t; # <---
        $s;
}); }
print "hello, world\n";
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However, if I remove the line marked with the arrow, the resulting code,

use Filter::Util::Call;
BEGIN{ filter_add(sub{
        my $s = filter_read;
        my $t = $_;
        $s;
}); }
print "hello, world\n";
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does not work anymore: specifically it does not print anything and exits with exit code 0.

I can't see why that statement makes a difference. Please explain.

Update 2008 jan 31: sent perl bug report.

Update 2008 feb 6: yes, sent it, but forgot to link to it. It's bug 50430.