Modules and packages are usually used interchangeably. But 
there is a difference.

a. Packages are perl files with .pm extn and is considered 
a separate namespace. So a package is nothing but group of 
related scalars,arrays,hashes and subroutines for a 
specific purpose.Once a package is included in a .plx file
(using "use") and you want to call one of the subroutines 
of the package, you may have to use the scope resolution 
operator &package::subroutine1 ( as the subroutine of the 
package is in a separate name space).

b. Modules are packages but which has the capabilities of 
exporting selective subroutines/scalars/arrays/hashes of 
the package to the namespace of the main package itself. So 
for the interpreter these look as though the subroutines 
are part of the main package itself and so there is no need 
to use the scope resolution operator while calling them.

This is usually done like:

use Exporter;
our @ISA=('Exporter');
our @EXPORT=('$x','@arr',subroutine)

(you are exporting a scalar, array and a sub-routine from 
the package). So if some .plx is using the above package 
they need not use the scope resolution to call these.
A direct access like "print $x" would work even without 
using the scope resolution.