#!/usr/bin/perl
use warnings;
use strict;
use DNAappend;
  my $a='ATGC';
  my $b='TTTT';
print"\n\n";
  print DNAappend($a,$b);
print"\n\n"; 
exit;

##</code><code>##

package DNAappend;
use strict;
use warnings;

sub DNAappend {
  my ($head,$tail)=@_;
  return ($head.$tail)
  }
1;

##</code><code>##

 C:\Users\x\Desktop>joindna.pl

Undefined subroutine &main::Dnaappend called at C:\Users\x\Desktop\joindna.pl line 8.

C:\Users\x\Desktop>