I haven't used Inline::C very much, so I'm not really familiar with its idiosyncracies. I would have expected this:

use Inline 'C' => <<CEND;
SV* x (HV* self) {
  SV** x_ref = hv_fetch( self, "x", 1, 0 );
  return (x_ref ? *x_ref : &PL_sv_undef);
}
CEND
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to work, but in fact it causes some nasty internal warnings:

Attempt to free temp prematurely: SV 0x816f1c4.
Attempt to free unreferenced scalar: SV 0x816f1c4.
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You'll only get the warnings if you have a DEBUGGING compile of perl, but they indicate a real problem! I'm not certain, but I would guess this is actually a bug in Inline::C. Update: it's actually a feature of XS – see rest of thread. Thanks sfink!

It seems that the following works correctly:

use Inline 'C' => <<CEND;
void x (HV* self) {
  SV** x_ref = hv_fetch( self, "x", 1, 0 );
  Inline_Stack_Vars;
  Inline_Stack_Reset;
  Inline_Stack_Push (x_ref ? *x_ref : &PL_sv_undef);
  Inline_Stack_Done;
}
CEND
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