More debugging... What's "wrong with the search" is the DN sent to Net::LDAP::search is wrong. I made two changes: Auth::ActiveDirectory Line 108:

    $self->{base} = qq/$self->{principal}/ unless $self->{base};
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Auth::ActiveDirectory Line 129::

    my $result = $self->_search_users( "sAMAccountName=$username" );
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... and now everything works quite well.