This was my final version of the algorithm. It benchmarked quickest of all the versions (7) that I tried, but I haven't found a simple way of verifying that it's a faithful re-implementation of the C version.

use constant A => 0;
use constant B => 1;
use constant C => 2;

sub mix ($$$) {
    use integer;
    $_[A] -= $_[B]; $_[A] -= $_[C]; $_[A] ^= ($_[C]>>13);
    $_[B] -= $_[C]; $_[B] -= $_[A]; $_[B] ^= ($_[A]<< 8);
    $_[C] -= $_[A]; $_[C] -= $_[B]; $_[C] ^= ($_[B]>>13);
    $_[A] -= $_[B]; $_[A] -= $_[C]; $_[A] ^= ($_[C]>>12);
    $_[B] -= $_[C]; $_[B] -= $_[A]; $_[B] ^= ($_[A]<<16);
    $_[C] -= $_[A]; $_[C] -= $_[B]; $_[C] ^= ($_[B]>> 5);
    $_[A] -= $_[B]; $_[A] -= $_[C]; $_[A] ^= ($_[C]>> 3);
    $_[B] -= $_[C]; $_[B] -= $_[A]; $_[B] ^= ($_[A]<<10);
    $_[C] -= $_[A]; $_[C] -= $_[B]; $_[C] ^= ($_[B]>>15);
}

use constant GOLDEN_RATIO    => 0x9e3779b9;
use constant INITHASH        => 2;

sub hash {
    use integer;
    my($a,$b,$c) = (GOLDEN_RATIO, GOLDEN_RATIO, $_[INITHASH]);
    my ($p, $len) = (0, length $_[KEY]);
    do {
        my($x,$y,$z) = unpack 'V3', substr($_[KEY] . (chr(0)x11), $p, 
+12);
        mix($a += $x, $b += $y, $c += $z + $len);
        $p+=12;
    } while $p <= $len;
    return $c;
}
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