Ah, ok. Since the bits Net::IMAP::Simple::Plus provides are irrelevant for pure authentication, and you still would prefer Net::IMAP::Simple::Plus, you could try to load Net::IMAP::Simple::Plus and fall back to Net::IMAP::Simple via a BEGIN block and string eval:

package Authen::Simple::LDAP;
BEGIN {
    eval "use Net::IMAP::Simple::Plus;";
    if ($@) {
        eval "use Net::IMAP::Simple;";
        die $@ if $@;
    }
}
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