This problem originally occured while running tests for installing 
Test::Harness, but I've been able to replicate it independently.  For 
reference, the original ticket is available at 
http://rt.cpan.org/NoAuth/Bug.html?id=6452.

The problem is that an local environment variable set in an anonymous block 
seems to be available to child processes created afterwards using qx().  I 
have not tested this using system() or fork().  See the example scripts 
below.

test1.pl

#!/usr/bin/perl -w

use warnings;
use strict;

my $key = "WANGO";

BLOCK1: {
    my $val = $ENV{$key};
    $val = "undef" unless defined $val;
    print "In BLOCK1, \$ENV{$key} = [$val]\n";
}


BLOCK2: {
    local $ENV{$key} = "FOTANGO";

    my $val = $ENV{$key};
    $val = "undef" unless defined $val;
    print "In BLOCK2, \$ENV{$key} = [$val]\n";
}


BLOCK3: {
    my $val = qx/test2.pl/;
    $val = "undef" unless defined $val;
    print "In BLOCK3, \$ENV{$key} = [$val]\n";
    print "   ... BUT IT SHOULD BE undef!" unless $val eq "undef";
}

=cut

test2.pl

#!/usr/bin/perl -w

use strict;
my $val = $ENV{WANGO};
$val = "undef" if ! defined $val;
print $val;

=cut

My results on Cygwin are...

In BLOCK1, $ENV{WANGO} = [undef]
In BLOCK2, $ENV{WANGO} = [FOTANGO]
In BLOCK3, $ENV{WANGO} = [FOTANGO]
   ... BUT IT SHOULD BE undef!


On Linux (Perl 5.8.4 on Gentoo with a 2.4.26 kernel), Win32 (ActiveState Build 
810), HP-UX (Perl 5.8.0), and Mac OS X (Perl 5.8.0), the results are...
In BLOCK1, $ENV{WANGO} = [undef]
In BLOCK2, $ENV{WANGO} = [FOTANGO]
In BLOCK3, $ENV{WANGO} = [undef]

Adding an "$ENV{WANGO} = undef;" to the end of BLOCK2 seems to take care of 
the problem, but coding this way for just one platform doesn't seem quite 
right to me.  I'm also not sure if this is actually a problem with Perl or an 
issue with Cygwin itself.

Regards,

Steve Peters
steve@fisharerojo.org