I understand how the problem manifests itself, it's not rocket science after all.

What I am saying is that if I want this:

sub foo {
   my $arg = shift;
   my $object = ($arg) ? Some::Package->new() : Other::Package->new();
}
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It seems blindingly obvious that the snippet you provided is not the way to go about it. I agree that the compiler should warn you about this (or better yet, disallow it entirely); it just seems that if someone has trouble with this, the problem probably lies with understanding 'my', not confusing if() syntax.