sub processA {
   ...
   commonInit()
   commonStuff( $various, $vars );
}
sub processB {
   ...
   commonInit()
   commonStuff( $various, $other, $vars );

}
{
    my @commonArray;

    sub commonInit {
        @commonArray = ();
    }

    sub commonStuff {
        # do stuff to @commonArray;
    }
}