if (defined $q->param("this_is_your_field_name")) {
    # do something spectacular
} else {
    $fieldempty = 1;
    # fail spectacularly
}
##</code><code>##

my @fieldnames = qw( name rank serialnumber haircolor favoritebeverage );
foreach my $field (@fieldnames) {
    $fieldempty = 1 unless (defined $fields{$field});
}
# do something spectacular here unless a field is empty