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