man GnuPG::UserAttribute (3): GnuPG User Attribute Objects

SYNOPSIS

# assumes a GnuPG::PublicKey object in $publickey
my $jpgs_size = $publickey->user_attributes->[0]->subpacket_total_size();

GnuPG::UserAttribute objects are generally not instantiated on their own, but rather as part of GnuPG::PublicKey or GnuPG::SecretKey objects.

new( %initialization_args ): This methods creates a new object. The optional arguments are initialization of data members;

validity: A scalar holding the value GnuPG reports for the calculated validity of the binding between this User Attribute packet and its associated primary key. See GnuPG's DETAILS file for details.
subpacket_count: A scalar holding the number of attribute subpackets. This is usually 1, as most UATs seen in the wild contain a single image in JPEG format.
subpacket_total_size: A scalar holding the total byte count of all attribute subpackets.
signatures: A list of GnuPG::Signature objects embodying the signatures on this user attribute.
revocations: A list of revocations associated with this User Attribute, stored as GnuPG::Signature objects (since revocations are a type of certification as well).

No useful information about the embedded attributes is provided yet. It would be nice to be able to get ahold of the raw JPEG material.