I would have thought that the two must be distinguished at compile time

A lexical var is either always an alias or never an alias, so you can for those. But it's a different story for globals.

sub func {
   $_ = "foo";
}

func() for my $var;   # $_ is an alias
func();               # $_ isn't an alias
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Update: Oops, not even for lexicals if you use Data::Alias. But you could say that if alias is being used, you know you're dealing with an alias.