From hv.h:

/* hash a key */
/* FYI: This is the "One-at-a-Time" algorithm by Bob Jenkins
 * from requirements by Colin Plumb.
 * (http://burtleburtle.net/bob/hash/doobs.html) */
/* The use of a temporary pointer and the casting games
 * is needed to serve the dual purposes of
 * (a) the hashed data being interpreted as "unsigned char" (new since
+ 5.8,
 *     a "char" can be either signed or unsigned, depending on the com
+piler)
 * (b) catering for old code that uses a "char"
 *
 * The "hash seed" feature was added in Perl 5.8.1 to perturb the resu
+lts
 * to avoid "algorithmic complexity attacks".
 *
 * If USE_HASH_SEED is defined, hash randomisation is done by default
 * If USE_HASH_SEED_EXPLICIT is defined, hash randomisation is done
 * only if the environment variable PERL_HASH_SEED is set.
 * For maximal control, one can define PERL_HASH_SEED.
 * (see also perl.c:perl_parse()).
 */
#ifndef PERL_HASH_SEED
#   if defined(USE_HASH_SEED) || defined(USE_HASH_SEED_EXPLICIT)
#       define PERL_HASH_SEED    PL_hash_seed
#   else
#       define PERL_HASH_SEED    0
#   endif
#endif
#define PERL_HASH(hash,str,len) \
     STMT_START    { \
    register const char * const s_PeRlHaSh_tmp = str; \
    register const unsigned char *s_PeRlHaSh = (const unsigned char *)
+s_PeRlHaSh_tmp; \
    register I32 i_PeRlHaSh = len; \
    register U32 hash_PeRlHaSh = PERL_HASH_SEED; \
    while (i_PeRlHaSh--) { \
        hash_PeRlHaSh += *s_PeRlHaSh++; \
        hash_PeRlHaSh += (hash_PeRlHaSh << 10); \
        hash_PeRlHaSh ^= (hash_PeRlHaSh >> 6); \
    } \
    hash_PeRlHaSh += (hash_PeRlHaSh << 3); \
    hash_PeRlHaSh ^= (hash_PeRlHaSh >> 11); \
    (hash) = (hash_PeRlHaSh + (hash_PeRlHaSh << 15)); \
    } STMT_END
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