I think I found my own answer. I wanted to use the current linux user's uid as part of a query (again, this is a perl/DBI script). I was thinking I might be able to set a variable into the DB session somehow using \set through DBI to accomplish this. The solution that finally occurred to me was to create a temporary table and have the perl script shove the uid into a column in that table, then query against that. Sort of like using a temp table to store variables set from outside.