for ('a'..'f') {
   my $letter = $_;  # Make modifiable copy
   ...
}

##</code><code>##

    foreach my $letter ('a' ... 'c', 'd' ... 'f') {
        print " $letter";
        $letter = uc $letter;
    }

##</code><code>##

#!/usr/bin/perl -w

use strict;
use warnings;

sub letters {
    foreach my $letter ('a' ... 'c') {
        print " $letter";
        $letter = uc $letter;
    }
    print "\n";
}

letters();
letters();
letters();