mark: seq_write

• seq_write is similar to seq_read:

 sub seq_write {
 my ($seqname, $data) = @_;
 my $newline = "$seqname: " . $data->run_list;

 for my $n (0 .. $#SEQUENCE) {
 for ($SEQUENCE[$n]) {
 if (/^\Q$seqname\E:\s+/) {
 $_ = $newline;
 return;
 }
 }
 }
 push @SEQUENCE, $newline;
 }

• Assigning to $_ (or doing push) modifies the file