post_install() {
if [ ! -r /usr/lib/perl5/site_perl/current/XML/SAX/ParserDetails.ini ]; then
    perl -MXML::SAX -e "XML::SAX->add_parser(q(XML::SAX::Expat))->save_parsers()"
fi
}

post_upgrade() {
if [ ! -r /usr/lib/perl5/site_perl/current/XML/SAX/ParserDetails.ini ]; then 
    perl -MXML::SAX -e "XML::SAX->add_parser(q(XML::SAX::Expat))->save_parsers()"
fi
}

op=$1
shift
$op $*