use strict;
use warnings;
use CGI;

my $cgi = new CGI;
my $name = $cgi->param('name');
my $email = $cgi->param('email');

%emails = ... # fetched from database I assume

if (defined $name && defined $email) {
  if (exists $emails{$email}) {
     # email exists in database already
  } else {
     # add email to system
  }
} else {
  if (defined $name) {
     # email is missing
  } elsif (defined $email) {
     # name is missing
  } else {
     # everything is missing
  }
}
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