In addition to the code above, you could use the -t filetest operator to check if STDIN was opened to a terminal (and thus not a pipe, etc): 

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

[download]
Or you could also try a non-blocking read of STDIN, which most directly answers your original question of seeing if there is any input waiting without blocking: 
  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.
  }

[download]
(Note that this technique may give false negatives if pipes take some time to start writing data.)

In reply to Re: Testing for the existence of STDIN? by wog
in thread Testing for the existence of STDIN? by darkomen

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