The perldelta for 5.8.1 contains this entry in "Future Directions":

A new feature called assertions will be available. This means that one can have code called assertions sprinkled in the code: usually they are optimised away, but they can be enabled with the "-A" option.

I'm not sure how this will look like in Perl, but I assume it would be something like:

ASSERT { stuff to check, things to warn about }
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The code block would not be compiled unless the -A flag is specified.

I have no idea how that would look in Perl6, but I would assume it would be similar to what 5.10 will bring.

Liz