use strict;
use warnings;


# set input file handle
my $fh;
if (scalar(@ARGV)) {
  # Note: 3 parameter open - this is much safer.
  my $file = $ARGV[0];
  open($fh, '<', $file) or die "Failed to open file $file: $!\n";
} else {
  $fh = \*STDIN;
}

# read in the data
my @hosts;
while (my $line=<$fh>) {
  # get rid of trailing new line on each hosts name
  chomp $line;

  # skip blank lines
  next unless $line;

  # add host to list of hosts
  push @hosts, $line;
}

# print it results
if (scalar(@hosts)) {
  # we have input so print it out
  local $"='|';
  print "hosts found: |@hosts|\n";
} else {
   # if @hosts is empty it isn't exactly a syntax error. Users will
   # be confused if you *only* print out the usage statement.
 
   if (scalar(@ARGV)) {
     # we read from a file but it was empty
     printf STDERR "file $ARGV[0] was empty!\n";
   } else {
     # oops no input! and we read from STDIN
     printf STDERR "I expected input from STDIN and got nothing!\n";
   }
   die <<__USAGE__;
usage: do_something [file]

...
__USAGE__
}