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#!/usr/bin/perl -wl
use strict;
...
    char *s = SvPV(bitstr, l);
    return popcount(s, l);
}

          Rate unpack popcnt
unpack  1252/s     --   -91%
popcnt 13581/s   984%     --

$ cat /proc/cpuinfo | grep name | head -1
model name    : AMD Phenom(tm) 9600 Quad-Core Processor

bits 64
global popcount
...
        mov rsp, rbp
        pop rbp
        ret