use warnings;
use strict;

  my $str = 'nnnnncattccattcattaattaattaatgaatgaatgnnnnn';
  while(<DATA>){
   chomp;   
     print $_,$str if !/^>|^$/;
 }

__DATA__
>contig number 11

tttgctcggaggggatc
>contig number 23

gaaaacacttccttattatacaggtaaaccgtatttggat
>contig number 3

aaagctcggaggggatcccct