You are seeing the bug in a form where it almost makes sense. However it really is a bug. When I first saw this, it was a bug in Number::Format which turned out to be due to a construct of this form:

  my $foo = $some_init unless test();
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The resulting bug is that if the test failed on 2 calls in a row, you had data from the first call of the function that polluted your output results. This bug was quite subtle and mysterious, and it turned out had been reported to the module author several times over the course of a year, but he was unable to figure out where the bug was.

Since then whenever I have looked at a large body of code for either of the patterns:

  /my .* if/
  /my .* unless/
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and found a match, it has usually resulted in also finding a bug. In short, this is a "feature" that is due to a broken optimization of Perl which, when it is hit in code, is almost always the cause of a bug in that code.

Now if you want static lexical variables without the usual "create a private scope" trick, that is doable in a fairly straightforward manner. In fact I just wrote and tested an implementation, which I just uploaded to CPAN. Hopefully Tie::Static will be accepted and become available shortly. (I intend to post the module to PerlMonks in a short while as well.)