Atomicity in DB transactions (Rollback related)

Hi,
I have a Pl/Perlu function in which I have a statement like this:

*******************
my $query_tbl_l_header = $dbh->prepare("SELECT
sp_insert_tbl_l_header(?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?)");

my $exec_l_from
=$query_tbl_l_header->execute($unmask_id,$from,$to,$sender,$subject,$replyto,$cc,$bcc,$messageid,$inreplyto,$reference,$mversion,$con_type,$con_id,$con_des,$con_enc,$con_length,$con_dis);
*******************

even if this execute, that calls a function sp_insert_tbl_l_header, fails,
subsequent trasactions continue without failing the whole perl function
there and then and makes the Db inconsistent.
Shouldn't the whole function fail and exit at that particular failure and
don't continue?
Please let me know how do these transactions work in postgres.

Thanks,
Jas