#!/usr/bin/perl
use warnings;
use strict;

my @values = 1 .. 70;
my @boundaries = (1, 10, 20, 40, 60);


my $OUT;
my $index = 0;
for my $value (@values) {
    if ($index <= $#boundaries && $value == $boundaries[$index]) {
        $index++;
        open $OUT, '>', "output.$index" or die $!;
        print {$OUT} "HEADER\n";
    }
    print {$OUT} $value, "\n";
}