Is webcal.cgi truly a CGI script? The problem is that in case of qx// or backticks, the script gets its parameters via @ARGV, and not through $ENV{QUERY_STRING} or STDIN with $ENV{CONTENT_LENGTH}.

Are you using CGI.pm to get parameters? If so, the script shouldn't have any problems parsing @ARGV.
The following works with ActivePerl

---------------cut foo.pl -------------------
use CGI;
my $q = new CGI;
print "$_=",$q->param($_),"\n" for $q->param;
---------------cut foo.pl -------------------

C:\Perl>perl -e "print qx/foo.pl foo=bar bar=foo/;"
foo=bar
bar=foo

C:\Perl>
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--perlplexer