If one input file is shorter than the other, it should return to the first line, which is the difficulty to me.

If you store the input files as arrays then it is less tricky:

#!/usr/bin/env perl 
#=====================================================================
+==========
#
#         FILE: zip.pl
#
#        USAGE: ./zip.pl input1 input2 output
#
#  DESCRIPTION: Zip 2 input files into one output file, looping the
#  shorter one.
#
# REQUIREMENTS: Path::Tiny
#        NOTES: See http://www.perlmonks.org/?node_id=1191418
#=====================================================================
+==========

use strict;
use warnings;
use Path::Tiny;

my @in = (
    path ($ARGV[0]),
    path ($ARGV[1])
);
my $out = $ARGV[2];

my @first  = $in[0]->lines;
my @second = $in[1]->lines;
my $max = $#first > $#second ? $#first : $#second;

open my $outfh, '>', $out or die "Cannot open $out for writing: $!";
for (0 .. $max) {
    print $outfh $first[$_ % @first] . $second[$_ % @second]
}

[download]

Bad argument trapping is left as an exercise.

In reply to Re: writing two files (different in length) to one output by hippo
in thread writing two files (different in length) to one output by ic23oluk

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