#!/usr/bin/env perl                                                     
                                                                        
use strict;                                                             
use warnings;                                                           
use List::Util qw(sum);                                                 
                                                                        
# For each $n, convert $n to hex digits, add the digits together. If the
# result has more than one hex digit, repeat this process adding the    
# digits together. When there is only one hex digit remaining, if the   
# digit is a '5', an 'a', or an 'f', the original number was divisible  
# by 5.                                                                 
                                                                        
for my $n (1..100) {                                                    
    my $h = sprintf '%x', $n;                                           
    while(length($h) > 1) {                                             
        $h = sprintf '%x', sum(map {hex $_} split //, $h);            
    }                                                                   
    print "$n is a multiple of 5\n" if $h =~ m/^[5af]$/;                 
}