-$helm_rdf_dir = "/usr/local/helm/rdf_library";
-$rdf_dbm = "/usr/local/helm/rdf_urls_of_uris";
-$rdf_indexname = "rdf_index.txt";
+my $cachemode = $ENV{'HTTP_GETTER_CACHE_MODE'} ||
+ 'gzipped';
+if (($cachemode ne 'gzipped') and ($cachemode ne 'normal')) {
+ die "Invalid HTTP_GETTER_CACHE_MODE environment variable, must be".
+ "'normal' or 'gzipped'\n";
+}
+my $helm_rdf_dir = $ENV{'HTTP_GETTER_RDF_DIR'} ||
+ "/usr/local/helm/rdf_library";
+my $rdf_dbm = $ENV{'HTTP_GETTER_RDF_DBM'} ||
+ "/usr/local/helm/rdf_urls_of_uris";
+my $rdf_indexname = $ENV{'HTTP_GETTER_RDF_INDEXNAME'} ||
+ "rdf_index.txt";