#Perl script to interleave two text files line by line

use strict;
die "Usage:\n  perl interleave.pl file1 file2 outfile\n"
  unless $ARGV[2];
open IN1, $ARGV[0] or die "Sorry, couldn't open $ARGV[0]. ";
open IN2, $ARGV[1] or die "Sorry, couldn't open $ARGV[1]. ";
open OUT, ">$ARGV[2]" or die "Sorry, couldn't create $ARGV[2].";

my ($in1, $in2);

#uncomment this next line to stop at end of shorter file
#print OUT "$in1$in2" while (($in1 = <IN1>) && ($in2 = <IN2>));

#uncomment this do { } block to continue to end of longer
#file, interleaving blank lines when the shorter file runs out. 
do {
  $in1 = <IN1> ;
  $in2 = <IN2> ;
  last unless ($in1 || $in2) ; #break when there's no more data
  $in1 = "\n" unless $in1;     #blank lines for absent data
  $in2 = "\n" unless $in2;
  print OUT "$in1$in2";
} until 0 ;