Crypt::OpenPGP::OnePassSig is a one-pass signature packet.
my $onepass = Crypt::OpenPGP::OnePassSig->parse($buffer);
Crypt::OpenPGP::OnePassSig implements a PGP One-Pass Signature packet, a packet that precedes the signature data and contains enough information to allow the receiver of the signature to begin computing the hashed data. Standard signature packets always come before the signed data, which forces receivers to backtrack to the beginning of the message--to the signature packet--to add on the signature trailer data. The one-pass signature packet allows the receive to start computing the hashed data while reading the data packet, then continue on sequentially when it reaches the signature packet.