if (-t STDIN) {
    # we're running from a shell, without input being
    # piped to us.
  } else {
    # ...
  }

##</code><code>##

  use IO::Handle;
  STDIN->blocking(0);
 # and try to read from it, like you would normally.
  my $line = <STDIN>;
  if (defined $line) {
    # we have input, read succeeded
  } else {
    # no input at the moment.
  }