Crypt::OpenPGP::Marker is a PGP marker packet.
my $marker = Crypt::OpenPGP::Marker->parse($buffer);
Crypt::OpenPGP::Marker is a PGP Marker packet. These packets are used by PGP 5.x to signal to earlier versions of PGP (eg. 2.6.x) that the message requires newer software to be read and understood.
The contents of the Marker packet are always the same: the three octets 0x50, 0x47, and 0x50 (which spell PGP).
It is very likely that you will never have to use a Marker packet directly.