package P;
use vars qw($f);
sub foo { print "$f\n" }

package main;
$P::f = "howdy, world";

P::foo();
__END__

output:
howdy, world

##</code><code>##

package P;
use strict;
use vars qw($f);  # NOTE: *f is a shared typeglob
sub foo {
    print $f,"\n";
    $f = "world";
}

package main;
use strict;
use vars qw($f);  # NOTE: *f shared with package P

*P::f = *f;    # make P::f an alias of main::f

$f = "howdy";
P::foo();      # after this call, $f is changed!
print $f,"\n";
__END__
ouput:
howdy
world