sub compare {
  return unless length $_[0] == length $_[1];

  # Omit if expecting differences almost all the time.
  return 1 if $_[0] eq $_[1];

  my $limit = 1;

  foreach (split('', $_[0]^$_[1])) {
     next unless ord;
     return 0 unless $limit--;
  }

  return 1;
}
[download]

{
   my @data = (
      [ qw( abab abab  ) ],
      [ qw( abab abaa  ) ],
      [ qw( abab qrst  ) ],
      [ qw( abab abba  ) ],
      [ qw( abab ababa ) ],
   );

   printf("%-5s -   %-8s is %sOK\n", @$_, (compare(@$_)?'':'not '))
      foreach @data;
}
[download]

Doesn't work with Unicode.