When Do You Report Semantics Errors?

I haven't commented on David Golden's work to allow references as the first operands to push and pop because I have mixed feelings about the feature. The simple explanation is that Perl 5.12 requires an explicit array as the first operand of both keywords:

push @some_array, $some_scalar;

my $other_scalar = pop @{ $some_array_reference };

With David's changes, you will also be able to write this code for Perl 5.14:

push $some_array_reference, $some_scalar;

my $other_scalar = pop $another_array_reference;

I like this change for a couple of reasons. First, it reduces the visual clutter of dereferencing. I've never cared for Perl 5's dereferencing syntax (it may be my least favorite syntactical part of the language), and David's right in that it's unnecessary in many cases here. Second, this change improves consistency in that all of these array manipulating functions obviously operate on containers, not the values of those containers. That is to say, pushing onto or shifting from an array modifies the contents of the array as a whole. It doesn't coerce those contents into a list and perform a transformation on the list. The consistency of writing, in effect, "It doesn't matter what kind of syntactic element represents an array—a bare array, a reference, or even something with tied array-like magic—as long as it behaves properly, that's the right thing to use here." appeals to me. (See also The Why of Perl Roles for an exploration of this design principle.)

Even so, I perceive a lessening of compile-time safety, even as I know that's probably an illusion.

If I write:

my @array = qw( some values here );
push @array, 'some other value';

... everything is fine. @array is obviously an array and is obviously correct. Yet if I write:

my @array = qw( some values here );
push @rray, 'some other value';

... then strict will catch the typo. @rray is obviously an array, but it's (probably) not present in the program at this point. So far so good. Yet if I write with 5.12:

my @array = qw( some values here );
push $array, 'some other value';

... that's obviously an error, because @array and $array are different variables. (The latter is most likely a typo, but that's just the peculiarly silly naming convention of this example, so don't count on it.)

With 5.14, I could also write:

my @array = qw( some values here );
push $hash_ref, 'some other value';

... and the error won't be visible until runtime, when push realizes its first operand isn't anything array-like at all. Of course, nothing prevents me from making a similar typo with Perl 5.12:

my @array = qw( some values here );
push @{ $hash_ref }, 'some other value';

... where the difference is a bit of extra (and, to my mind, somewhat ugly) syntax.

I know this isn't a peculiar disadvantage of the change, and I like the change overall, but it still seems to me to trade a little bit of compile-time safety for the potential for run-time errors. I'll use it and I'll get used to it and I'm certain I'll like it, especially with complex data structures. Perhaps it's my familiarity with explicit deferencing that gives the illusion of compile-time safety.

(You can use Vincent Pit's autovivification pragma to avoid some of the potential damage of mistakes with explicit or implicit dereferencing, but again that's not a compile-time fix. I suspect what I really want here is Perl 6's gradual typing system.)