use IO::Select;

# Get a list of hosts from either a file or STDIN
my @hosts;
if($#ARGV != -1) {
  my $file = $ARGV[0];
  if(! -f $file) { die "File $file doesn't exist\n" }

  open FH, "<$file" or die "Failed to open file $file: $!\n";
  @hosts = <FH>;
  close FH;
} else {
  my $sel = IO::Select->new();
  $sel->add(\*STDIN);

  my @ready = $sel->can_read(0);
  if($#ready == -1) {
    die <<__USAGE__;
usage: do_something [file]

...
__USAGE__
  }
  @hosts = <STDIN>;
}