From the perlguts manual:

The hash algorithm is defined in the "PERL_HASH(hash, key, klen)" macro:

hash = 0;
while (klen--)
  hash = (hash * 33) + *key++;
hash = hash + (hash >> 5);              /* after 5.6 */
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The last step was added in version 5.6 to improve distribution of lower bits in the resulting hash value.

If you just want a look at the hashing function, this is it. A look at the full source might give you a better feel for it. If you want to call this from perl, you can use Inline::C which automatically includes perl.h. Alternatively, you can reimplement this function in pure perl and use it from there.