my $foo = 1 does stuff in two parts:

• $foo is declared at compile time
• $foo is initialised at run time

That's the reason for your contradictory error messages

Here's what perlsyn has to say about it:

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.