hi,
it's not too mutch to ask. in fact i'm relay glad to explain some code, 'cause it shows, that you are willing to learn. so here it comes:

### my recursive subroutine
###########################
sub enum {

    ### return the list of elements (aka nucleotides) if this is the l
+ast
    ### recursion.
    ### also note that --$_[0] decrements the counter _before_ it is t
+ested
    ##################################################################
+#####
    return @{$_[1]} unless --$_[0];

    ### define a variable which holds my return values
    ##################################################
    my @return;

    ### iterate over the return values of the next recursion
    ########################################################
    for ( enum( $_[0], $_[1] ) ){
        my $nuc = $_;

        ### iterate over all nucleotides
        ################################
        for ( @{$_[1]} ) {
            push @return, $nuc . $_;
        }
    }

    ### return the list so far to the prior recursion
    #################################################
    return @return;
}
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if you have problems to understand how it works, try to think about what happens, when $_[0] = 1 then think about what happens, when $_[0] = 2 and then you should get it.
if you don't understand this at all, it could be because my english is not even close to perfect. so maybe i just didn't explain it correct, which means "don't hesitate to ask questions" :-)