Its a feature (of sorts) and it's unrelated to weaken(). When an anonymous sub is compiled, a "prototype" sub (CV) is created. At run time, whenever 'sub {...}' is executed, a copy of that prototype CV is made, as you would expect.

However, as an optimisation (with unfortunately visible side effects), if that CV isn't closing over any outer lexicals, the CV isn't copied, it's shared: so its ref count is bumped and a new reference to it is returned. Which is what you're seeing here.

The other common visible side-effect is this:

for (1,2) {
    push @a, sub {};
    my $lex;
    push @b, sub { $lex };
}

bless $a[0];
bless $b[0];
print "$a[1] $b[1]\n";
[download]

which outputs

main=CODE(0x1f41648) CODE(0x1f40c88)

Whoops, $a[1] is blessed too.

No-ones thought of a way round this yet, but removing the optimisation would make simple anon subs a lot slower.

Dave.