$s1 = "\0";
$s2 = $s1 x 2;
my $diff_pos = ($s1 ^ $s2) =~ /[^\0]/ ? $-[0] : undef;

print defined $diff_pos 
      ? "diff_pos says the strings differ at $diff_pos\n" 
      : "diff_pos says the strings are equal.\n";
print $s1 eq $s2 
      ? "and Perl says they are equal\n"
      : "and Perl says they are not equal\n";