#!/usr/bin/perl
BEGIN {
        $SIG{__WARN__} = sub { print "Warn in package ".caller().": ",  @_ };
    package X;
    $foo = "I'm in X, export me!";
    *Y::foo = *foo
}

package Y;
sub foo { "Oh dear.. I wasn't meant to be outside Y" }
warn $foo;

package X;
warn foo();

__END__
Warn in package Y: I'm in X, export me! at t.pl line 11.
Warn in package X: Oh dear.. I wasn't meant to be outside Y at t.pl line 14.