ambrus has asked for the wisdom of the Perl Monks concerning the following question:

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.

Replies are listed 'Best First'.
Re: Wierd behaiviour of Filter::Util::Call
by BrowserUk (Patriarch) on Jan 29, 2008 at 21:08 UTC

    Some sort of deep doodoo in the XS voodoo. Perhaps the following code? 

    # Call.XS ~line 130
    SAVESPTR(DEFSV) ;/* save $_ */
    /* make $_ use our buffer */
    DEFSV = sv_2mortal(newSVpv("", 0)) ;

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    A maybe workaround is to quote uses of $_, which seems to prevent the bug: 

    use Filter::Util::Call;
BEGIN{ filter_add(sub{
        my $s = filter_read;
        my $t = "$_";
        $s;
}); }
print "hello, world\n";

    [download]
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
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