interleave.pl

#!/usr/bin/env perl

use strict;
use warnings;

my $count = shift
  or die "Usage: $0 <file count>\n";

my @files = map { open my $file, ">", "part$_.out" or die "open: $!"; $file; } 1..$count;

$files[($. - 1) % @files]->print($_) while <>;