why does plperl cache functions using just a bool for is_trigger

I see that plperl uses a triple of (function oid, is_trigger flag, user
id) as a hash key for caching compiled functions. OTOH pltcl and plpgsql
both use (oid, trigger relation oid, user id). Is there any reason why
just using a bool as plperl does would be wrong?

I'm trying to write a validator function for plpython and I'm not sure
if I should copy plperl's or plpgsql's logic.

Cheers,
Jan