Being able to specify an arrayref so that you can pass user-defined additional parameters is a good idea anyway, but the need for it could be obviated in many cases. The following should be enough:

use Class::CGI
    handlers => {
        invoice_date => 'My::Date::Handler',
        sales_date   => 'My::Date::Handler',
    };
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This can work if you just pass the parameter name to the handler constructor. Users can then create a convention based on the name and avoid the need for configuration:

package My::Date::Handler;

use Carp;
use My::Date;

sub new {
    my $class = shift;
    my ( $cgi, $param_name ) = @_;

    $param_name =~ s/_date\z//
        or croak "Profile name expected to end in '_date': $param_name
+";

    my %date =
        map { $_ => scalar $cgi->raw_param( $param_name . '_' . $_ ) }
        qw( month day year );

    return My::Date->new( %date );
}

1;
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Makeshifts last the longest.