So the simple answer is to avoid having both sides of the 'x' operator as constants. For example, this will fix your situations:

my $VAR = makeA(20000000);
sub makeA { return 'A' x (shift) }
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This would also fix it:

my $VAR = make20m('A');
sub make20m { return (shift) x 20000000 }
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The conclusion seems to be that perl the constant itself occupies memory, and then the variable gets its own copy of the constant.