How to do blue-green deployment with PL/SQL code?
Yes, it is possible, but it takes a bit of work. I assume that the concern with duplicating the database is 1) time and 2) size/storage, otherwise it seems like it would be trivial to spawn databases for testing against. Based on the fact that you are targeting PL/SQL i'm guessing you are working with Oracle.
You will either need: 1) a smaller-scale dataset that can act as a representative sample of you data which is sufficient for testing all of your corner cases or 2) to clone the whole production database. Honestly, I recommend option #2. There are numerous cases where things operate correctly at a smaller scale, but run into issues once scaled up - especially with database data. Ideally, you could store and replay a sample of queries for load testing using a database proxy like ScaleArc (if Oracle won't let you store your queries).
Saying you need to clone your whole dataset may make me sound like a kook and like it will take forever, but you may find this answer in which I detail a methodology for cloning multi-terabyte databases in seconds without significantly increasing your storage costs due to duplication and delta-snapshotting useful. This may be harder if you are using Oracle's proprietary storage system. Be forewarned, the tech isn't cheap, but if you wanted cheap, you wouldn't be using Oracle. The best way to sell that to your upper management is to prove that the tech is cheaper than the outage/data mangling it would prevent. It's your insurance policy against outages - so how much insurance are you willing to buy?