I would avoid words like "de facto, default" and just do my own checks:

linux perlbrew:

perl -V | grep -i multi
      usemultiplicity=undef
[download]

linux system perl:

    usemultiplicity=define
Compile-time options:
    ...
    MULTIPLICITY
[download]

I liked Multiplicity - what the heck is it?

bw, bliako