pl/perl interpreter instance(s) - how long does it persist?

Hi,
Apologies in advance if this has been covered before, but I've searched
extensively without finding anything so far.

I would like to know how long the pl/perl interpreter instances persist?

I ask because I'm doing some work where we have PL/Perlu code that loads
some Perl modules in trigger code. If I change the underlying modules
that have been loaded, then when can I expect the new version to take
effect?

Initial testing seems to give me contrary results.
In some cases, creating a new DB then loading the same module in a new
stored procedure will get the old version.
But in some other cases, the new DB will pick up the new version of the
module.

I wondered if this might be due to the backend having a number of perl
interpreters in different processes, and I'm getting a fresh process
sometimes, and sometimes I'm getting an old one that's already loaded
the modules.

So.. I'm a bit confused and hoped I could come and ask for advice.

Thanks,
Toby