# Backend::A
package Backend::A;
sub foo { }
1;

##</code><code>##

# Backend::B
package Backend::B;
sub foo { }
1;

##</code><code>##

# main
my $backend = 'Backend::A';
require $backend;
$backend->foo();

# then later, if necessary:
$backend = 'Backend::B';
require $backend;
$backend->foo();