my @set = 'A' .. 'Z';
my $subset_size = 5;
my @subset = qw/A M T X Z/;

process_subset(@subset);
process_subset(@subset);

sub process_subset {
    if (seen(@_)) {
        print "Already processed '@_'\n";
        return;
    }
    print "Performing expensive operation on '@_'\n";
    set_seen(@_);
}

sub seen {
    # For you to implement
    # ...
}

sub set_seen {
    # For you to implement
    # ...
}