trouble making PG use my Perl

We're trying to upgrade our Pg 9.6 installs up to 12.2. In a break from tradition where we grab source and compile our own, I've downloaded the community RPMs for Centos 6 and installed them (they default into /usr/pgsql-12 it seems).

I can make Pg come up, initdb, that sort of stuff just fine. But we also use the Perl extension and we have references to Perl modules that are in *our* Perl and not the system one. Yes, we compile our own Perl like we provide our own Pg because Centos uses much older versions.

The issue is that I've not been able to make Pg use our Perl (in /opt/perl) instead of the system one (in /usr). I've tried changing the env-vars in multiple places, the most obvious being the /etc/init.d script, but none of that helps. When we compiled our own Pg, I could set this with "export PERL=/opt/perl/bin/perl" before the "configure --with-perl" command. Setting PERL in the init.d file doesn't help either.

Sadly, I can't find anything about this in the docs. Searching online doesn't turn up anything either as the search terms are too generic (or I can't find a way to make them specific enough to help). Haven't found any options for postgresql.conf either.

So how does one set the Perl binary/library at server start time?
Am I going to have to compile my own Pg again to make this work?

Thanks,
Kevin
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