Thank you. This has proved extremely useful. I've been able to replace a swathe of LWP code with

$result=qx(curl -X POST --data "$query" 'https://www.paypal.com/cgi-bi
+n/webscr');
[download]

and it seems to work.

The bit that is now confusing me is how to replace LWP's

is_error(status) and is_success(status)

There is barely a mention of response codes in the curl documentation.

LATER...

Thanks to a search of StackOverflow, I now have a working workaround that give me access to the response code

read (STDIN, $query, $ENV{'CONTENT_LENGTH'});
$query .= '&cmd=_notify-validate';
 my ($head,$body) = split( m{\r?\n\r?\n}, qx(curl -X POST --data "$que
+ry" -si 'https://www.paypal.com/cgi-bin/webscr'),2 );
 my ($code) = $head =~m{\A\S+ (\d+)};
[download]