I've not really used Mojo, but I think you probably want something like this...

app->attr(dbh => sub {
   my $self = shift;
   my $dsn  = 'dbi:Oracle:schema';
   my $usr  = 'Scott';
   my $pas  = 'tiger';
   my $dbh  = DBI->connect_cached( $dsn, $usr, $pas );
   return $dbh;
});

app->attr(cached_query => sub {
   my $self = shift;
   my $sql  = 'SQL QUERY HERE';
   return $self->dbh->prepare_cached( $sql );
});

# later

get '/query' => sub{
   my $c   = shift;
   my $dbh = $c->app->dbh;
   my $sth = $c->app->cached_query;
   
   # probably pull @values out from GET query or POST data
   my @values = do { ... };
   
   my $results = $dbh->selectall_arrayref($sth, {}, @values);
   
   # and so on
};
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