Perl doesn't have a clue what blah_seq.nextval means. Actually, neither does your sql environment, but it doesn't care: it just hands it over to the database server to deal with.

$sth=$dbh->prepare("
insert into blah (blah_key, blah_name)
values(blah_seq.nextval, ?)");
$sth->execute($blah_name);
[download]

ought to do what you want.