no warnings 'regexp'; ($str0 ^ $str1) =~ /^((??{chr(0)})*)/;
$commonLen = $+[0];

##</code><code>##

($str0 ^ $str1) =~ /^\0*)/;

##</code><code>##

#!/usr/bin/perl -l

use strict;
use warnings;

sub clen {
    ($_[0] ^ $_[1]) =~ /^\0*/;
    $+[0];
}

print +(clen split) while <DATA>;
__END__
aaaa bbbb
aaaa abbb
aaaa aabb
aaaa aaab
aaaa aaaa