HELP_MESSAGE() and exit unless $ARGV[1];
foreach (@ARGV) {
  HELP_MESSAGE() and exit unless /^-?\d+$/
} 

# ... and later ...
sub HELP_MESSAGE {
  print $help;
}

##</code><code>##

die $help unless $ARGV[1];
map { /^-?\d+$/ or die "Invalid input $_\n" } @ARGV;