Ah, ikegami already posted the answer... in a ticket in the perl bug tracker dave_the_m(?) says

a (more or less unfixable) design flaw in perl's internals: the stack not being ref counted.

and shows this code example

%h = qw(
  a 1
  b 2
);
f(%h);

sub f {
  %h = qw(c 3); # uh oh :-(
  use Carp;
  Carp::confess();
}
__END__
 at /home/davem/tmp/p line 10.
    main::f("a", "", "b", 3) called at /home/davem/tmp/p line 5
[download]

which is akin to those ikegami posted.

For perl v5.26 the output is slightly different:

 at baz.pl line 10.
    main::f("a", "", "b", "") called at baz.pl line 5
[download]

This might be due to other fixes than the underlying issue, which is still there.