This is not a bug. If Perl behaved in a way contrary to defined behavior, that's a bug. But failure to behaive how you expect, having not read (or remembered) the perldocs, doesn't constitute a bug.

From perlsub:

NOTE: The behaviour of a my statement modified with a statement modifier conditional or loop construct (e.g. my $x if ...) is undefined. The value of the my variable may be undef, any previously assigned value, or possibly anything else. Don't rely on it. Future versions of perl might do something different from the version of perl you try it out on. Here be dragons.

The way it does behaive when attached to an 'if' modifier is simply a side-effect of how my is implemented, but as the perldocs state, it's a side effect that you should not count on, and a construct that you shouldn't use; "Here be dragons."