#!/usr/bin/perl -w
use strict;
use IO::File;

#interleave files, first argument is line count, rest are files.

my $lc = shift;
my @fhs;

foreach (@ARGV) {
  my $fh = new IO::File;
  open ($fh, "<$_");
  push @fhs, $fh;
}

while (--$lc) {
  foreach (@fhs) {
    print scalar(<$_>);
  }
}
[download]

Tested minimally but the path is clear from here I hope.

--
$you = new YOU;
honk() if $you->love(perl)

use FileHandle;

my @files  = qw( 1.txt 2.txt 3.txt);
my @handles = map{ 
     $_ = new FileHandle $_, 'r';
     die "Could not open $_: $!" unless defined($_);
     $_;
   } @files;
open MIXED, ">$mixed_file" or die "Could not open $mixed_file: $!";

while( @handles ){
  foreach my $fh (@handles){
     my $line = $fh->getline or undef $fh;
     print MIXED $line if defined $fh;
  }
  @handles = grep defined, @handles;
}
close MIXED;
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Jeroen
"We are not alone"(FZ)

1. The open call for the MIXED file did not have a '>'. This is sheer carelessness.
2. I was openning the files to the same typeglob, which resulted in three references to the same open file being stored in the array. This is a more subtle bug.
3. I was testing for eof after reading in a line, instead of before. This would result in the last line never being written to the mixed file.
4. Finally, I was using 'last' within the inner loop, foolishly thinking it would get me out of the outer loop. Another dumb mistake.

my @files = ("1.txt", "2.txt", "3.txt");
my @fhs;

for ($i = 0; $i < scalar(@files); ++$i) {
     open($fhs[$i], $files[$i]) || die;
}
open(MIXED, ">mixed.txt") || die;
OUTER:
while (1) {
     foreach $fh (@fhs) {
         last OUTER if (eof($fh));
         $line = <$fh>;
         print MIXED $line;
     }
}
map { close($_); } @fhs;
close(MIXED);
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-ton