This may be your problem.
From the perlguts manpage:

Note that both `hv_store' and `hv_store_ent' do not increment the reference count of the stored `val', which is the caller's responsibility. If these functions return a NULL value, the caller will usually have to decrement the reference count of `val' to avoid a memory leak.

Maybe create the SV with:

SV *val = sv_newmortal();
[download]

Or mortalize it with:

sv_2mortal(val);
[download]

Might work? You might need to Mortalize 'key' too.