You can't avoid having a variable as global if it is used in global scope. The first use does not necessarily imply the scope of a variable, as you found out.

I would possibly rewrite your code as follows:

sub get_response {
  my ($res) = @_;
  die "Required argument missing" unless $res;
  if ($res =~ /Hello/i) {
    return "Hello to you.\n";
  } else {
    return "Huh?\n";
  };
};

print get_response(@ARGV);
[download]