Here's an example taken from Maplat::Web::MemCache.pm:

    my $memd;
    my $memd_loaded = 0;
    # Decide which Memcached module we want to use
    # First, we try the festest one, then the standard
    # one and if everything fails we use our own
    my $memdtype;
    {
        ## no critic (BuiltinFunctions::ProhibitStringyEval)
        if(eval('require Cache::Memcached::Fast')) {
            print "    Cache::Memcached::Fast available.\n";
            $memdtype = "Cache::Memcached::Fast";
            $memd = Cache::Memcached::Fast->new ({
                            servers   => [ $self->{service} ],
                            namespace => $self->{namespace} . "::",
                            connect_timeout  => 0,
                        });
            $memd_loaded = 1;
            $self->{mctype} = "fast";
        } elsif(eval('require Cache::Memcached')) {
            print "    No Cache::Memcached::Fast ... falling back to C
+ache::Memcached\n";
            $memdtype = "Cache::Memcached";
            $memd = Cache::Memcached->new ({
                            servers   => [ $self->{service} ],
                            namespace => $self->{namespace} . "::",
                            connect_timeout  => 0,
                        });
            $memd_loaded = 1;
            $self->{mctype} = "slow";
        } else {
            print "    No Cache::Memcached* available ... will try to 
+use Maplat::Helpers::Cache::Memcached\n";
        }
    }
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This just looks up which Memcached modules are available and tries to load one of those.