my $cgi = CGI->new;
if ($cgi->param ('foo') {
  # have data to process
} else {
  # no data, so print an empty form
}

##</code><code>##

if ($ENV{QUERY_STRING} || 'POST' eq $ENV{REQUEST_METHOD}) {
  # have data to process
} else {
  # no data, so print an empty form
}