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use CGI 'param'; # use the CGI module, and import the function named "
+param"

$email_to = param('extra_emails'); # this assumes that your text input
+ is called "extra_emails"

my %checkboxes = (
ChkBoxName1 => 'EmailGroup1@Domain.com',
...
foreach my $checkbox_name (keys %checkboxes) {
  $email_to .= "$checkboxes{$checkbox_name}, " if param($checkbox_name
+); # if the checkbox was ticked, the param call returns true, and $em
+ail_to gets the email address added.
}