I agree with Jenda here, actually. Whether the code written for the older version runs on the newer one is not really the determining factor in my mind.

What counts is that idiomatic Perl 5 is very different from idiomatic Perl 4 in architecture, whereas idiomatic Perl 6 won’t be nearly as different from idiomatic Perl 5, at least in everyday code that does not strain against the limitations of Perl 5 too hard. Perl 4 was really more of a scriptable tool, whereas Perl 5 is a serious language.

Perl 6 does not really reform the way systems written in Perl are to be architectured, it just makes these architectures easier to implement by putting various and sundry premanufactured, well-designed nuts and bolts into the language, so you don’t have to spend so much time building them all yourself.

In syntax, it is a much bigger departure from Perl 5 than Perl 5 was from Perl 4; but in spirit, Perl 6 is much closer to Perl 5 than Perl 5 is to Perl 4.

Makeshifts last the longest.