If you're using CGI.pm (and you should), you can check to see if a field is empty with the following code:

if (defined $q->param("this_is_your_field_name")) {
    # do something spectacular
} else {
    $fieldempty = 1;
    # fail spectacularly
}
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Since most code that doesn't use CGI.pm (and you should be using it) at least puts fields in a hash, you can store your fieldnames in an array and still do right:

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
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