use strict;
use warnings;

open(IN, "<", "rep_set_ass_tax.fna") or die "Died!!"; 
open (OUT , ">","seq_id.txt") or die "Cannot open output.txt: $!";
 while (my $line=<IN>) { 
   if($line=~/>/) {
 
      my @vettore=split(" ", $line); 
    
      foreach (@vettore){ 
       print OUT $_ . "\n"; 
      }
   } 
 }
 
 close IN;
 close OUT;