#! /usr/bin/perl
use strict;
use Getopt::Std;
use Symbol;
use vars qw(@handles $opt_f);
getopts('f:');
if ($opt_f) {
  open (STDOUT, ">$opt_f") or die "Cannot write to $opt_f: $!";
}
unless (@ARGV) {
  die "Usage: mergefile [-o<output_file>] file1 file2";
}
foreach my $file (@ARGV) {
  my $fh = gensym;
  # Letting someone pipe from a cmd is useful enough to allow
  # having someone try to wipe out a file with ">".
  open ($fh, $file) or die "Cannot read from $file: $!";
  push @handles, $fh;
}
while (@handles) {
  my $fh = shift @handles;
  if (defined( my $line = <$fh> )) {
    print $line;
    push @handles, $fh;
  }
  # else $fh goes out of scope here and autocloses
}