$ grep -r 'XML::LibXML::SAX' /usr/share/perl5/
/usr/share/perl5/vendor_perl/XML/SAX/ParserDetails.ini:[XML::LibXML::SAX::Parser]
/usr/share/perl5/vendor_perl/XML/SAX/ParserDetails.ini:[XML::LibXML::SAX]
/usr/share/perl5/vendor_perl/XML/SAX.pm:has a driver called XML::LibXML::SAX::Generator, which is used in place of

$ pacman -Qo /usr/share/perl5/vendor_perl/XML/SAX/ParserDetails.ini
error: No package owns /usr/share/perl5/vendor_perl/XML/SAX/ParserDetails.ini

So I deleted (moved) ParserDetails.ini & get_iplayer worked without perl-xml-libxml, but with the error message:
could not find ParserDetails.ini in /usr/share/perl5/vendor_perl/XML/SAX

I reinstalled perl-xml-sax & it replaced ParserDetails.ini but with a truncated version:
$ diff /usr/share/perl5/vendor_perl/XML/SAX/ParserDetails.ini /tmp/ParserDetails.ini
3a4,14
> [XML::SAX::Expat]
> http://xml.org/sax/features/namespaces = 1
> http://xml.org/sax/features/external-parameter-entities = 1
> http://xml.org/sax/features/external-general-entities = 1
>
> [XML::LibXML::SAX::Parser]
> http://xml.org/sax/features/namespaces = 1
>
> [XML::LibXML::SAX]
> http://xml.org/sax/features/namespaces = 1
>

Now it works with no error message