I know that there is a statement about this in one of the perl docs, although I cannot find it now.

From perlsyn:

 NOTE: The behaviour of a "my" statement modified with a statement mod
+ifier conditional or loop construct (e.g. "my $x if ...") is undefine
+d.  The value of the "my" variable may be "undef", any previously ass
+igned value, or possibly anything else.  Don't rely on it.  Future ve
+rsions of perl might do something different from the version of perl 
+you try it out on.  Here be dragons.
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So the following:

my $v2 = -2 if 0;
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Will have undefined and probably unexp(?:exted|licible) behavior. Don't do it! Don't use a my assignment with a conditional!

Define your variable first, then set it using the conditional.