Thank you Laurent, BrowserUk, AnomalousMonk, and Ken for the explanations and reassurances :-)

It does certainly make sense that the memory allocated for the localized data structures is subject to Perl's usual memory management, that is basically what I was hoping for, but couldn't immediately confirm, so thank you for your explanations. Here's another quick test I did:

use Devel::Refcount qw/refcount/;
my $x;
our %h = (one=>'two');
{ local %h = (three=>'four');
  $x = \%h;
  print "$x ", refcount($x), "\n"; }
print "$x ", refcount($x), "\n";
__END__
HASH(0x2fb246a) 2
HASH(0x2fb246a) 1
[download]