package Foo;
use strict;
use warnings;

sub Hello_World { print "Hello Word\n"; }
my $callback = sub { Hello_World(); };
sub Bar {
    $callback->();
    print "This is the actual 'would be' exported sub\n";
}
1;

# Some script that uses it
#!/usr/bin/perl
use strict;
use warnings;
use Foo;
Foo::Bar();