#file: tee.pl
use strict;
use warnings;
$| = 1;      # **** turn off output buffering *****

sub usage () {
print "TEE USAGE:\n tees stdout to a file and to stdout\n".
      " program | tee outfile\n".
      " sends stdout from program to outfile\n";
      exit;
}


my $filename = shift @ARGV;
usage unless $filename;

open (OUTFILE, ">>$filename") || die "Can't open OUTFILE: $!";

while (<>){
  print;
  print OUTFILE;
}