my $infile = "rep_set_ass_tax.fna";
my $outfile = "seq_id.txt";
open my $in,  '<',  $infile  or die "Can't read $infile; $!\n"; 
open my $out, '>>', $outfile or die "Can't read $outfile; $!\n"; 
while (my $line=<$in>) { 
    if($line=~/^>/) {          
        my @vettore=split(/\s+/, $line);  
    print $out "$vettore[1]\n";
    } 
}