#!/usr/bin/perl -w
use strict;
use Symbol;
use Text::Tabs;
die("usage: overlay file1 file2\n") unless scalar @ARGV == 2;
my @data;
for(my $i=0; $i < scalar @ARGV; $i++){
  open($data[$i]->{FH}=gensym, $ARGV[$i]) || die("overlay($ARGV[$i]): $!\n");
  1 while(readline(*{$data[$i]->{FH}}));
  $data[$i]->{LINE} = $.;
  seek($data[$i]->{FH}, 0, 0);
}
my $maxline = $data[0]->{LINE} > $data[1]->{LINE} ?
  $data[0]->{LINE} : $data[1]->{LINE};
for(my $i=0; $i < $maxline; $i++){
  if( $i > $data[0]->{LINE} ){
    while(readline(*{$data[1]->{FH}})){
      print;
    }
    last;
  }
  elsif( $i > $data[1]->{LINE} ){
    while(readline(*{$data[0]->{FH}})){
      print;
    }
    last;    
  }
  else{
    for my $data (@data){
        $data->{str} = readline(*{$data->{FH}});
        chomp($data->{str});
        $data->{str} = expand($data->{str});
    }
    my $maxchar = length($data[0]->{str}) > length($data[1]->{str}) ?
        length($data[0]->{str}) : length($data[1]->{str});
    my @onechars = split(//, $data[0]->{str});
    my @twochars = split(//, $data[1]->{str});
    my $str;
    for(my $j=0; $j<$maxchar; $j++){
      if( $j > $#onechars ){
        $str .= join('', splice(@twochars, $j));
      }
      elsif($j > $#twochars){
        $str .= join('', splice(@onechars, $j));
      }
      else{
        $str .= $onechars[$j] eq ' ' ? $twochars[$j] : $onechars[$j];
      }
    }
    print $str, "\n";
  }
}