puzzled

[Pedro Antonio Reche]

Hi there, I am puzzled by the 'build_seq' subroutine in following code.
#!/usr/sbin/perl -w
use strict;
use vars qw($USAGE);
# random sequence generator #
# -c=1 option will cause prot sequences to be built
# using vertebrate aa frequencies,
# with option -a putting a 1st methionine residues on. Frequencies are
# calculated from the NCBI human RefSeq protein sequences
# -c and -a only affect protein sequences
# -a only works in conjunction with -c
# -n number of random sequences, default = 1
use Getopt::Long;
my ($length,$type,$filename,$comp,$met);
$USAGE = 'usage: generate_random_seq.pl --length=1000 --type=dna 
--filename=/tmp/test.seq --number=50';

my %alphabets = ( 'dna' => [qw(C A G T)],
                 'rna' => [qw(C A G U)],
                 'prot'=> [qw( A C D E F G H I K L M N P Q R S T V W Y)],
             );
# make random num from 1-10000. numbers in this array reflect the frequency,
# e.g., a random number from 1.744 = A, 745-991 = C etc;
my @aa_frequencies = qw(744 991 1398 2017 2378 3104 3349 3726 4239 5273 
5443
                       5749 6410 6848 7455 8263 8760 9340 9488 9713 10000);
my $number = 1;

&GetOptions
 (
  'l|length:s'          => \$length,
  't|type|m|alphabet:s' => \$type,
 );
assert ( $type && defined ($alphabets{lc $type}),
        $USAGE);
assert ( $length && $length =~ /^\d+$/, $USAGE );
foreach my $num (1..$number) {
  my $sequence = "";
  my $alphabet = $alphabets{lc $type};
  my $sspace = scalar @$alphabet;
  if (!$comp || $type ne 'prot') {
     foreach ( 1..$length ) {
        $sequence .= $alphabet->[ int rand($sspace) ];
     }
  }elsif ($type eq 'prot') {
     ($sequence) = build_seq($length, [EMAIL PROTECTED]);
       print "yes $sequence\n\n\n";
  }
   print "$sequence\n";
}
sub assert { die $_[1] unless( $_[0] ); }
sub build_seq {
  #takes seqlen and ref to frequency data as parameters
  my ($len, $pf)  = @_;
  my $str;
  my  $i;
  $i = 2; #1
  for ($i .. $len-1) {
     print "$i\n";
         my $aa = int(rand (10000)) ;
     my $j = 0;
     while ($pf->[$j] < $aa && $j <19) {
        $j++;
     }
     $str .= $alphabets{'prot'}[$j];
  }
  print "str is $str\n"; #2
  #return $str; # 3
}
I run the program as
./program.pl --type prot --length 9
and the program spit out a random string of 9 characters such as
KWNNNPITS
However, the string should be of 7 characters ($i = 2 in # 1), print 
does not work within the subroutine (#2). and subroutine is returning a 
value despite return is commented out (#3). Does anyone understand why 
this is working this way?
Thanks in advance for any insight.
Cheers

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