#!/usr/bin/perl -w

use strict;
use warnings;

open(my $fh, '<', '/tmp/list') or die "Unable to open file, $!";

my @entire_file=<$fh>;
for my $i (0..$#entire_file) {
my @group;
push @group,$entire_file[$i]  until ( $i % 5 == 0 );
`tool.pl @group`;
}