use warnings;
use strict;
{
	package Debug;
	use Exporter 'import';
	our @EXPORT = qw/debug/;
	our $FLAG = 0;     # package variable
	sub debug {
		if ($FLAG) {
			print @_;
		}
	}
}
{
	package Foo;
	Debug->import;     # normally "use Debug;"
	$Debug::FLAG = 1;  # global enable
	debug("Hello\n");  # prints "Hello"
}
{
	package Bar;
	Debug->import;      # normally "use Debug;"
	$Debug::FLAG = 0;   # global disable
	debug("World!\n");  # doesn't print
}