sub foo {
   my ($foo) = @_;
   print($foo->{bar}, "\n");
}

##</code><code>##

sub foo {
   my %foo = @_;
   print($foo{bar}, "\n");
}

##</code><code>##

HV* hv = newHV();
SV* rv = newRV_noinc(hv);  /* noinc to abandon our ref to the HV */

... populate the hash using hv_store ...

...

XPUSHs(sv_2mortal(rv));   /* Mortalize to abandon our ref to the RV */

...