Yes this is the case; Through libraries we are trying to make it easy to CALL perl5 (and ruby and python and...) modules from Perl 6. But porting -- aka translating -- is another matter. Mostly it requires a similar amount of effort as if you are translating your program to any other dynamic language for nontrivial programs.

My understanding is that LOTS of Python2 programs work fine in Python3 with no (or very trivial) changes. Very very few Perl5 programs will work fine in Perl6 with no changes.

While this is true, a good deal of porting could be managed by packaging up old code into Perl5 modules and using Perl6 to glue it together, no?