If you need help in this department, write a Perl::Critic rule that checks whether you make a copy as the first line of your map/grep/for/foreach/sub block. Your suggestion would mean slowing down almost any scalar assignment done.

For lexicals, couldn't the check be done at compile-time?

chomp (my $module = shift);
require $module or die;
for my $foo (@array) {
    func($foo);
}
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How are you going to determine at compile time whether $foo gets modified? Because if func changes its $_[0], the elements of @array get modified.

OK. I can just about imagine how assignment to an ordinary scalar and assignment via an alias would be the same at run-time -- at run-time it doesn't much matter where the pointer to the SV has come from.

However, I would have thought that the two must be distinguished at compile time -- something has to take care of the implied dereference -- for no run-time penalty.

I realise that there are plenty of other traps for the new and the unwary -- but I can still remember the pain of learning about this one !

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.