This is defined in hv.v of the perl5.005_03 source. It looks like a variation on Daniel J. Bernstein's comp.lang.c "Times 33 with addition" hash function:

#define PERL_HASH(hash,str,len) \
     STMT_START    { \
    register char *s_PeRlHaSh = str; \
    register I32 i_PeRlHaSh = len; \
    register U32 hash_PeRlHaSh = 0; \
    while (i_PeRlHaSh--) \
        hash_PeRlHaSh = hash_PeRlHaSh * 33 + *s_PeRlHaSh++; \
    (hash) = hash_PeRlHaSh; \
    } STMT_END
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This is defined in hv.v of the perl-5.7.2 source. The article mentioned in the comment is here.

/* FYI: This is the "One-at-a-Time" algorithm by Bob Jenkins */
/* from requirements by Colin Plumb. */
/* (http://burtleburtle.net/bob/hash/doobs.html) */
#define PERL_HASH(hash,str,len) \
     STMT_START    { \
    register const char *s_PeRlHaSh = str; \
    register I32 i_PeRlHaSh = len; \
    register U32 hash_PeRlHaSh = 0; \
    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
[download]

For anyone who is interested, these hashing functions are also implemented in Ralf Engershall's Str library.

--
John.