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use strict;
use vars qw($foo);
# ... code that uses $foo ...
1;

use strict;
use Library;
# You can now use $foo freely.

use vars qw($foo);
# Some code here.

package Bar;
# Now you can't use $foo, and won't accidentally access $main::foo.

package Foo;
use strict;
...
our @EXPORT_OK = qw(foo);
# etc
1;

package Foo;
use strict;
...
@EXPORT_OK = qw(foo);
# etc
1;

package Foo;
use Exporter qw(import);
...
use strict;
# etc
1;