SYNOPSIS
# Functional style
use Digest::HMAC_MD5 qw(hmac_md5 hmac_md5_hex);
$digest = hmac_md5($data, $key);
print hmac_md5_hex($data, $key);
# OO style
use Digest::HMAC_MD5;
$hmac = Digest::HMAC_MD5->new($key);
$hmac->add($data);
$hmac->addfile(*FILE);
$digest = $hmac->digest;
$digest = $hmac->hexdigest;
$digest = $hmac->b64digest;