Hello, I am fairly new to Perl and Perlmonks so I hope I'm not breaking any rules by posting this message here! I was having a look at some algorithms on Bioperl (http://www.bioperl.org/wiki/Getting_all_k-mer_combinations_of_residues). Basically, out of the four letters 'A', 'T', 'G' and 'C', the following code gives me all the combinations of k-long "words" (so if k = 3, it would produce AAA, AAT, AAG... CCC):

use warnings; use strict;

 sub kmers {
     my $k = shift;
     my $alphabet = qw( A T G C );
     my @bases = @$alphabet;
     my @words = @bases;
     for ( 1 .. --$k ) {
         my @newwords;
         foreach my $w (@words) {
             foreach my $b (@bases) {
                 push (@newwords, $w.$b);
             }
         }
         @words = @newwords;
     }
     return @words;
 }
[download]

I can understand most of this code, but I have trouble understanding how the

loop is being implemented. Whenever I've seen these for/foreach loops, once INSIDE the loop, you usually do something with the list of numbers inside the parenthesis (in this case, numbers 1 through 'k minus one'). In this case, I cannot see how the list is being used. I have read the Perldoc entry on foreach loops, but I couldn't find out how this loop works.

Could someone please refer me to a resource/reference that explains how such a loop might be working? Again, I'm sorry if this seems to be too trivial compared to most of the questions asked on this site.