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

##</code><code>##

MacQIIME Mac-Pro-di-Francesca:cartella senza titolo $ perl taxa.pl 
syntax error at taxa.pl line 6, near ");"
syntax error at taxa.pl line 8, near "$fh;"