In my ideal world, I'd have /(lib path) in the lib already, and then use dirAAA::ModuleA and dirBBB::ModuleB.

Assuming, however, that you're really using "use ModuleA" to get at ModuleA, you can use %INC:

my $libpath;
BEGIN {
  ($libpath = $INC{'ModuleA.pm'}) =~ s:/dirAAA/.*:dirBBB:;
}
use lib $libpath;
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