As was suggested by Corion at the time, you are probably better off with a hash for this.

Here is a self-contained example that demonstrates:

#!/usr/bin/perl -l
use strict;
use warnings;

my $string = 'P,F,';

my %letters = map { $_ => 1 } split /,/, $string;

for (qw(P F T)) {
    print "$_ is ", $letters{$_} ? "set" : "not set";
}
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Which prints:

P is set
F is set
T is not set
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Cheers,
Darren :)