Image::Identiconを参考にした。

use Digest::SHA qw(sha1);
my $ip = "172.16.84.33";

my $num = join '', split /\./, $ip;

my $code = unpack("N",sha1($num));

my $blue  = (($code >> 6) & 0x0ff);
my $green = (($code >> 15) & 0x0ff);
my $red   = (($code >> 24) & 0x0ff);

print $blue,",",$green,",",$red;

saltをまぶしたほうがよいのかな。