use Data::Dumper;

my $unqchar5_regex = qr/^(?{%_=()})(?:(.)(?{$_{$1}++}))+
                         (??{(scalar(keys %_)<5)?~$1:''})$/x;

my $pswd = "abcdef";
print (($pswd =~ $unqchar5_regex)?"Pass\n":"Fail\n");
print Dumper \%_;

my $pswd = "abcdbd";
print (($pswd =~ $unqchar5_regex)?"Pass\n":"Fail\n");
print Dumper \%_;