%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