around at either build-time or at run-time is:
USE_TOOLS+= perl # build-time
USE_TOOLS+= perl:run # run-time
Also remove some places where perl5/buildlink3.mk was being included
by a package Makefile, but all that the package wanted was the Perl
executable.
This makes all of the buildlink files come after any variable definitions
that could affect their behaviour, and makes all of the target
definitions come after all of the variables and .ifdef logic. This
matches the way that many of the large, complex packages are already
written.
