Use of uninitialized values in a list - arrays

Question

I load some variables from the URL and want to check in my CGI script whether are they initialized. I quickly found out that I can not put uninitialized values into an list in Perl.
My approach was simply to iterate through the list and use a switch variable $sucessDOM (1 if all data had been initialized and 0 elsewhere).
I came up with this:
### Read data from HTML
my $mailinglist = param( 'mailinglist' );
my $firstname = param( 'firstname' );
my $lastname = param( 'lastname' );
my $mail = param( 'mail' );
my $sucessDOM = 0;
my #DOM = { $mailinglist, $firstname, $lastname, $mail };
### Data validation
for my $data ( #DOM ) {
if ( $data eq undef ) {
$sucessDOM = $sucessDOM + 1;
}
if ( $sucessDOM = #DOM ) {
print "#DOM\n";
$sucessDOM = 1;
}
else {
$sucessDOM = 0;
}
}
The output is as follows
[Fri Dec 22 20:30:40 2017] read_form.cgi: Use of uninitialized value $mailinglist in anonymous hash ({}) at ./read_form.cgi line 20.
[Fri Dec 22 20:30:40 2017] read_form.cgi: Use of uninitialized value $lastname in anonymous hash ({}) at ./read_form.cgi line 20.
[Fri Dec 22 20:30:40 2017] read_form.cgi: Use of uninitialized value in string eq at ./read_form.cgi line 24.
HASH(0x55c8a73eeea8)
I also cannot understand why are there errors for only two values in the array if they are all uninitialized, as I run the script directly without passing any values.

Answer 1

You have at least four problems with your code:
First, {} makes an anonymous hash and returns a reference to it; this is a single value that ends up in #DOM. The warnings are because hash keys have to be strings, and warn if they are undef. You should be assigning like
my #DOM = ( $mailinglist, $firstname, $lastname, $mail );
Second, eq compares strings, so it would convert undef to ''. To test if $data is undef, do
if (! defined $data) {
Third, you are checking if all the array elements were successful inside the for loop (where that will not be true on any but the last iteration of the loop). Move the if ($successDOM... block to after the for loop.
Fourth, = is just an assignment operator; use == for numeric comparison:
if ($successDOM == #DOM) {