• Lexical variables can do the closures trick. Package variables can’t.
• Perl 5′s default is package variables. We know lexical variables to be the better default, but we’re bound by backwards compatibility in Perl 5. In Perl 6, the default is lexical variables.

So, it’s not that Perl 6 beats Perl 5 in terms of features; they’re equally powerful in this regard. But Perl 6 has the saner default.

#!/usr/bin/perl

use 5.010_000;

sub counter {
my ($count) = @_;
return sub { $count++ };
}

my $c1 = counter(5);
say $c1->(); # 5
say $c1->(); # 6

my $c2 = counter(42);
say $c2->(); # 42
say $c2->(); # 43

I thought about mentioning in the post an occasion a few years ago at which I thought I had found a really serious bug in perl. In analyzing it, I applied all the techniques i use with Perl 6 of reducing the problem to a one-liner to isolate and identify the problem. At the end, I was left with a call to &sort and one more use of $a. And I went “oh” and remembered the exception about $a and $b.

The package-globalness of $a and $b is understandable from a historical perspective — $a and $b were used in &sort as package globals before lexical scoping was introduced — but it’s one of the things that Perl 5 got wrong and can’t back-compat itself out of. Perl 6 fixes the whole problem without much fanfare.