Doesn't occur for perl v5.8.7 built for i486-linux-gnu-thread-multi. 
Warning: something's wrong at test.pl line 7.
Warning: something's wrong at test.pl line 7.
ab

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perl588delta says the following:

There has been a fair amount of refactoring of the C source code, partly to make it tidier and more maintainable. The resulting object code and the perl binary may well be smaller than 5.8.7, in particular due to a change contributed by Dave Mitchell which reworked the warnings code to be significantly smaller. Apart from being smaller and possibly faster, there should be no user-detectable changes.

...maybe that's introduced this weird behaviour...

In reply to Re: Strange warn behavior in loop by rpanman
in thread Strange warn behavior in loop by deibyz

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