Are you sure you want combinations? There is a difference between combinations and permutations and you also need to consider replacement and duplicates. I think the following code, adapted from How To: Make An Iterator does what you want.

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

# Change this as desired
my $size = 2;

my $next_perm = fix_size_perm($size, 'a'..'z', 0..9);
while ( my $perm = $next_perm->() ) {
    print "$perm\n";
}
sub fix_size_perm {
    my ($size, @list) = @_;
    my @curr = (0) x ($size - 1);
    push @curr, -1;
    return sub {
        if ( (join '', map { $list[ $_ ] } @curr) eq $list[ -1 ] x @cu
+rr ) {
            @curr = (0) x (@curr + 1);
        }
        else {
            my $pos = @curr;
            while ( --$pos > -1 ) {
                ++$curr[ $pos ], last if $curr[ $pos ] < $#list;
                $curr[ $pos ] = 0;
            }
        }
        return undef if @curr > $size;
        return join '', map { $list[ $_ ] } @curr;
    };
}
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Update: Changed code example to hopefully better fit the problem.