The reason is because $foo = 20 doesn't occur until after you've called all those functions.

DECLARATION occurs at compile-time. ASSIGNMENT happens at run-time. Different things. my $foo = 20 has a compile-time effect (my) and a run-time effect ($foo = 20).

For your desired action, you'd need something like:

my $foo;
BEGIN { $foo = 20 }
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