my $foo = new Foo("bar");

package Foo;
sub new {
    my $class = shift;
    my $var = shift;
    bless \$var, $class;
}

##</code><code>##

---------(In Foo.pm)------------------
package Foo;
sub import {
    $Foo::var = shift;
}
---------(In a different file)--------
use Foo qw(bar);  # Sets $Foo::var to "bar"
#...
$Foo::var = "baz"; # Changes $Foo::var to "baz"