Simple check that every parameter is alphanumeric

my %params = CGI::Vars;
foreach my $p (keys %params) {
  unless ($params{$p} =~ /[[:alpha:]]/) {
    push @error_params, $p
  }
}

print "these params failed: ", join(", ", @error_params);
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If you know how many parameters to expect, you could count the number of parameters which were set:

my %params = CGI::Vars;
if (scalar(keys %params) < $no_expected) {
  print "error, you missed something.";
}
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