sub revdnacomp {
  # my $dna = @_;  
  # the above means $dna gets the number of 
  # arguments in @_, since it's a scalar context!

  my $dna = shift; # or   my $dna = shift @_;
  # ah, scalar context of scalar gives expected results.
  # my ($dna) = @_; # would work, too

  my $revcomp = reverse($dna);
  $revcomp =~ tr/ACGTacgt/TGCAtgca/;
  return $revcomp;
}
[download]

sub revcompl { # operates on all elements passed in
  my (@dna) = @_;
  foreach my $segment (@dna) {
    my $revcomp = reverse($segment);
    $revcomp =~ tr/ACGTacgt/TGCAtgca/;
    push @done, $revcomp;
  }
  return @done; # or reverse @done;  # what's best semantics?
}
[download]

sub revcompl {
    return map { (my $rev = reverse $_) =~ tr/ACGTacgt/TGCAtgca/; $rev
+ } @_
}
[download]

Makeshifts last the longest.

The above sub does not work