One way to handle this is to use a Factory pattern, where the product of the Factory is an object whose type depends on the packet content.

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.