#!/usr/bin/env perl
#
# allcharstwice - print the lines on which each character occurs exactly twice
#
# $Id: allcharstwice 663 2009-10-01 19:10:51Z rp $
#
# Example of use: words -x * | grep '[aeiuo']' | uniqer | allcharstwice

use warnings;
use strict;

while (<>)
{
  /\S.*\S/
   or next;
  my %occ;
  map { ++$occ{$_} } split( //, $& );

  if ( !grep { $_ ne 2 } values %occ )
  {
    print;
  }
}