#earlier in SQL
CREATE TABLE tbl ( id serial, col2 text, col3 text );
NOTICE:  CREATE TABLE will create implicit sequence 'tbl_id_seq' for SERIAL column 'tbl.id'

##</code><code>##

my $myid;
my $dbh=DBI->connect("DBI:Pg(AutoCommit=>0,RaiseError=>1):dbname=foo");
eval {
    $dbh->do("INSERT INTO tbl ( col2, col3 ) VALUES ( 'val', 'val3' )";
    my $sth=$dbh->prepare("SELECT currval('tbl_id_seq')");
    $sth->execute;
    $sth->bind_columns( \($myid ));
    $sth->fetch;
    $dbh->commit;
};

if($@)
{
    $dbh->rollback;
    #enlightening error message goes here
}

# $myid has your id in it here.