You can use polymorphism to get the varying behavior of these objects.

One simple way to do it is this:

package Packet;

my %patterns;    # could add some in declaration

sub addType {
    my $regex     = shift;
    my $className = shift;
    $patterns{$className} = $regex;
}

sub removeType {
    my $className = shift;
    delete $patterns{$className};
}

sub makeOnePacket {

    # get a packet
    my $packet = getAPacket();
    return undef unless $packet;
    my $newObject;
    my ( $className, $regex );
    while ( ( $className, $regex ) = each(%patterns) ) {
        my @fields;

        if ( @fields = ( $packet =~ $regex ) ) {
            $newObject = $className->new( $packet, @fields );
            last;
        }
    }
    $newObject;
}

# These two must be defined by classes that want
# to work with this system.

sub newFromPacket {
    my $class  = shift;
    my $packet = shift;
    my @fields = @_;

    # initialize as needed, return object.
    bless { packet => $packet, fields => \@fields }, $class;
}

sub process {
    my $self = shift;

    # now do whatever is necessary
}

package main;

Packet::addType( qr/^(abc)(.*)/, 'SomeType' );
Packet::addType( qr/^(def)(.*)/, 'SomeOtherType' );

while ( my $newPacket = makeOnePacket() ) {
    $newPacket->process();
}
[download]

Note that now you can add new packages that work with this system by just including them. Stick the following in NewPacket.pm:

package NewPacket;
use Packet;

# Could have inherited from Packet, but no
# reason to. Luckily, this isn't Java or C++.
sub newFromPacket {
    my $class  = shift;
    my $packet = shift;
    my @fields = @_;

    # initialize as needed, return object.
    bless { packet => $packet, fields => \@fields }, $class;
}

sub process {
    my $self = shift;
}

# you could also use __PACKAGE__ here:
Packet::addType( qr/^(something)(.*)/,     'NewPacket' );
Packet::addType( qr/^(somethingElse)(.*)/, 'NewPacket' );    # returns
+ true
[download]

Then a simple use NewPacket; in your main will add the new functionality.

update: added explanation about adding new types.