When using ThingISBN alone you get a warning : Failed Can't use string ("1") as a HASH ref while "strict refs" in use at /home/koha/src/C4/XISBN.pm line 99. sub get_xisbns { my ( $isbn, $biblionumber ) = @_; my ($response,$thing_response,$syndetics_response,$errors); ... $response->{isbn} = [ @{ $syndetics_response->{isbn} or [] }, @{ $thing_response->{isbn} or [] } ]; Vars $thing_response,$syndetics_response must be initialised as an hashref. Or maybe rewrite a bit this code.