Variables in Postgresql

Hi.

I haven't seen any documentation about this, how do you use variables in a
simple way in postgresql? I want to use same sequence number in a dynamic
query from Perl.

When i'm using MSSQL i can do like this in a single query

"DECLARE @owner INT
INSERT maintable (fields) VALUES (...)
SELECT @owner = @@IDENTITY
INSERT subtable (fields) VALUES (@owner, ...)"

How does I save a value and using it later in the query..?

kind regards
Tomas