Immediate fix:

While priv_key() in Crypt::DH::GMP does not work as a setter (but is supposed to; that's the bug), priv_key can be set in the constructor. So changing:

    $DH = Crypt::DH::GMP->new;

    $DH->g($g);
    $DH->p($p);
    $DH->priv_key($DH_private);
    $DH->generate_keys;
[download]

to:

    $DH = Crypt::DH::GMP->new(p        => $p,
                              g           => $g,
                              priv_key => $DH_private);

    $DH->generate_keys;
[download]

...solves the problem, and is tidier code anyway.