As per brother mifflin above, from the Perl Cookbook ^{1st ed, pp 518, 519}...
  hth,
  ybiC

Use -t to test STDIN and STDOUT:

sub I_am_interactive {
  return -t STDIN && -t STDOUT;
}
[download]

If you're on a POSIX system, test process groups:

sub I_am_interactive {
  local *TTY; # local file handle
  open(TTY, "/dev/tty") or die "can't open /dev/tty: $!";
  my $tpgrp = tcgetpgrp(fileno(TTY));
  my $pgrp = getpgrp();
  close TTY;
  return ($tpgrp == $pgrp);
}
[download]