Thanks to the advice I have received, I have made the following modifications which I will include for future reference to anyone who may have had the same questions as me... thanks dhoss and perrin!

Controller

sub create :Local{
   my ( $self, $c ) = @_;

   my $nation = $c->request->params->{nation};

   #Action Logic; Form was submitted, lets create then
   #Check to see if the nation name already exists
   #UPDATED: create method overloaded to return 0 if
   #nation name exists

   if($nation) {
       if($c->user->nations->create({
                                     name  => $nation,
                                     owner => $c->user->get('id'),
                                     map_nation_resource => [ { resour
+ce_id => 1 }, { resource_id => 2 } ],
                                     })){
               $c->redirect($c->uri_for('/nation'));
       }
       else {
               $c->stash->{error_msg} = "Nation name already taken!";
       }
   }

      return $c->stash->{template} = 'nation/create.tt2';
}
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ResultSet subclass

package Game::DB::ResultSet::Nation;
use strict;
use warnings;
use base 'DBIx::Class::ResultSet';

#Override nation's create method to check if nation name exists
#Before creating. If it does, return 0

sub create {
    my $self = shift;
    my $nation) = shift;

    if($self->find( {name => $nation->{name}} ) ) {
        return 0;
    }

    $self->next::method($nation);
}

1;
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