This isn't really interpolation. You are initializing $World with a soft reference to it. strict objects to that. Even without strict, you get a syntax error if you try to declare $World as aq lexical that way. It reads,

$ perl -e'my $var = "World";my ${$var} = "hello";'
Can't declare scalar dereference in my at -e line 1, near "} ="
Execution of -e aborted due to compilation errors.

$ perl -e'my $var = "World";${$var} = "hello";print $World'
hello$
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