I think you should keep existing definitions of Foo and Bar. There's no reason to change them. I might implement a FooBar class like this though:

package FooBar;
our @ISA = qw/Foo Bar/;

sub new {
    my ($class, %args) = @_;
    bless {
        map %$_,
        $class->Foo::new(%args),
        $class->Bar::new(%args),
    }, $class;
}
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and use it like this:

package main;

my $a = FooBar->new(name => 'Zoid', age => 55);
printf "name: %s, age: %s\n", $a->name, $a->age;
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As long as there are no naming conflicts, this should be good. If there are naming conflicts you might choose to override the access method.

Also I don't like your directory structure. I usually go down the inheritance hierarchy which only works for single inheritance. Truck/FireTruck.pm. If FireTruck is both a Truck and a Fire, I wouldn't name it FireTruck/Fire.pm and FireTruck/Truck.pm. You may find that you can think of one base class its true base and the other is like a role. But better yet put them all at the same directory level.