#!/usr/bin/perl -w
use strict;

my @dls = 'dl01'..'dl20';  # what you start out with

my $nusers = 5;
my $vars;

for (my $i = 0; $i < @dls; $i += $nusers) {
    push @{ $vars->{Dates} },
        { EndDates => [ @dls[$i..$i+$nusers-1] ] }; 
}

use Data::Dumper;
print Dumper $vars;

__END__
$VAR1 = {
          'Dates' => [
                       {
                         'EndDates' => [
                                         'dl01',
                                         'dl02',
                                         'dl03',
                                         'dl04',
                                         'dl05'
                                       ]
                       },
                       {
                         'EndDates' => [
                                         'dl06',
                                         'dl07',
                                         'dl08',
                                         'dl09',
                                         'dl10'
                                       ]
                       },
                       {
                         'EndDates' => [
                                         'dl11',
                                         'dl12',
                                         'dl13',
                                         'dl14',
                                         'dl15'
                                       ]
                       },
                       {
                         'EndDates' => [
                                         'dl16',
                                         'dl17',
                                         'dl18',
                                         'dl19',
                                         'dl20'
                                       ]
                       }
                     ]
        };