Have you determined which of the values it is complaining about (is it maybe RootName => undef, or $data as LanX pointed out)? Have you tried any alternatives, i.e. making sure you're not feeding the function undefs where you shouldn't be? Note the effect of warnings is lexical in scope, so it won't change anything coming from inside of XML::Simple. If that is the case, and you're feeding the function valid data, you could try the following ugly hack to suppress all warnings from inside XMLout (although it's about as ugly as using XML::Simple for XML output in the first place...): my $xmlout = do { local $SIG{__WARN__}=sub{}; XMLout(...) };

Minor edits for clarification.