use warnings;
no warnings 'redefine';
use strict;

use lib '.';
#use Count;

print One::foo();
{
    my $count = Count->new;
    my $bar = $count->mock;
    #print One::foo();
}
print One::foo();

BEGIN{
package One;

sub foo {
    return "foo\n";
}
1;

package Count;

sub new {
    return bless {}, shift;
}
sub unmock {
    my $self = shift;
    *One::foo = \&{ $self->{sub} };
}
sub mock {
    my $thing = shift;
    my $self;

    if (ref($thing) eq __PACKAGE__){
        $self = $thing;
    }
    else {
        $self = bless {}, __PACKAGE__;
    }

    $self->{sub} = \&One::foo;
    *One::foo = sub {
        use Scalar::Util 'weaken';
        weaken $self;
        $self->{x} = 'x'; return "baz\n";
    };
    return $self;
}
sub DESTROY {
    my $self = shift;
    print "destroying...\n";
    $self->unmock;
}
1;

}
[download]

foo
destroying...
baz
[download]