Curiosity killed the cat (meow)

I found the null strings, which come from the undefined captures ($2 is not found, and undefined) and I guess $_ gets set to a null string if it is equated to an undefined capture?...

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

my $str = '129-129A & B-131 NORTH AV';

print "\nDon't check for undefined matches\n";
while ($str =~ m[(.*?)(?:\s*([&/+-])\s*|\s+)]g){
    print "\$1='$1'\t\t\$2='$2'\n";
}

print "\nCheck for undefined matches\n";
while ($str =~ m[(.*?)(?:\s*([&/+-])\s*|\s+)]g) {
    my $one = "undef"; my $two = "undef";
    $one = $1 if defined $1;
    $two = $2 if defined $2;
    print "\$1='$one'\t\t\$2='$two'\n";
}

print "\nPseudo Split\n";
my @b = ($str =~ m[(.*?)(?:\s*([&/+-])\s*|\s+)]g,$');
foreach (@b) {
   $_="undef" unless defined $_;
   print "'$_',\n";
}
[download]

Result (NB: Active state does not give me the warning messages, although I get them on Solaris)

Don't check for undefined matches
$1='129'                $2='-'
$1='129A'               $2='&'
$1='B'                  $2='-'
Use of uninitialized value in concatenation (.) or string at test.pl l
+ine 8.
$1='131'                $2=''
Use of uninitialized value in concatenation (.) or string at test.pl l
+ine 8.
$1='NORTH'              $2=''

Check for undefined matches
$1='129'                $2='-'
$1='129A'               $2='&'
$1='B'                  $2='-'
$1='131'                $2='undef'
$1='NORTH'              $2='undef'

Pseudo Split
'129',
'-',
'129A',
'&',
'B',
'-',
'131',
'undef',
'NORTH',
'undef',
'AV',
[download]

UPDATE: I was actually trying to comment on Brower UK's post, but slipped up. Oh well