use strict;
use warnings;

my $str = q{abcdceefghfty};

for my $ch (qw(a b c d e f g))
{
    print qq{"$ch" Unique in "$str"\n}
       if $str !~ m{($ch)(?=.*\1)};
}

##</code><code>##

"a" Unique in "abcdceefghfty"
"b" Unique in "abcdceefghfty"
"d" Unique in "abcdceefghfty"
"g" Unique in "abcdceefghfty"

##</code><code>##

use strict;
use warnings;

my $str = q{abcdceefghfty};

for my $ch (q{a} .. q{z})
{
    my $matchTxt
       = q {\A}
       . qq{[^$ch]*($ch)}
       . q {(?!.*\1)};
    my $rxUniq = qr{$matchTxt};
    print qq{"$ch" unique in "$str"\n}
       if $str =~ $rxUniq;
}