#!/usr/bin/perl

    use strict;
    use warnings;
    
    my @numbers = qw(
        1
        10
        100
        1000
        10000
        100000
        1000000
        10000000
        100000000
        1000000000
    );

    # Test
    print "Number: " . form_num($_, '.' ) . "\n" for @numbers;
    print "----US\n";
    print "Number: " . form_num($_, ',' ) . "\n" for @numbers;
    
sub form_num {
    my $num = reverse(shift);
    my $sep = shift;
    
    $num =~ s/(\d{3})(?=(?:\d{1,3}))/$1$sep/g;    
    reverse($num);
}