If you have XML::SAX (which includes a PurePerl parser) then no external libraries are required for XML::Simple to work. If you want better parsing speed then by all means use XML::Parser (or XML::SAX::ExpatXS) but either of these Perl modules will need access to libexpat. No other external libraries are required.
If I were trying to diagnose it, I'd probably use the strace tool (on Linux) to log the system calls that the application is making. The log would probably show an attempt to open a file that doesn't exist.
The other thing you could do is rebuild your PerlApp compilation with a modified copy of XML::Simple. Find the line that generates that error and modify the error string to include $@ - the text of the error that caused the parser load to fail. E.g.:
croak "XMLin() requires either XML::SAX or XML::Parser ($@)"; 