There is a simple way. Instead of checking if a password looks like a username, you can first translate the username into this sort of "code" and then check if they look the same. Something along the lines of

my %codes = (
  l => 1, L => 1,
  i => 1, I => 1,
  z => 2, Z => 2,
  e => 3, E => 3,
  h => 4, H => 4,
  s => 5, S => 5,
  G => 6, g => 9,
  t => 7, T => 7,
  b => 8, B => 8,
  o => 0, O => 0
  );
my $user = 'pileofdung';
my $translated;

for (split //, $user) {
    $translated .= (defined $codes{$_}) ?  $codes{$_} : $_;
}

print $translated,$/; # p1130fdun9

my $password = 'p113.0f.dun9%';

my $match =0;
for (split //, $translated) {
    $match++  if $password =~ /$_/
}
print "they match\n" if $match >= length($password) -2;
# you can choose how lax you want to be by
# setting an appropriate number of characters that
# you want to be different between username and password
# in this case if all but 2 characters are the same, it
# is a bad password
[download]

Of course, you can use any other comparing methods, but just to give you some ideas to play with.