The point was that if you actually use
the ability to interoperate between .NET and Perl, then you have used something that is specific to the Microsoft implementation of .NET which is not supported by other versions of the CLR.
Therefore that is a non-portable thing that is easy to take advantage of. (And is one of Microsoft's platform lock-in tricks even while they make a great song and dance over being platform independent.)
Now if you use Visual Studio to write Perl and you don't interoperate with .NET in the resulting code, then you have no portability problems. But if you use the PerlNet component, expecting to take advantage of the features that Microsoft lists (including portability), then you have fallen for a bit of a sham.