Each package contains a special array called @ISA which contains a list of that class's parent classes, if any. This array is simply a list of scalars, each of which is a string that corresponds to a package name. The array is examined when Perl does method resolution, which is covered in perlobj.
To load packages while adding them to @ISA, see the parent pragma. The discouraged base pragma does this as well, but should not be used except when compatibility with the discouraged fields pragma is required.