local $\="\n";

sub HUMANo {
    my $n = shift;
    my $c = 3;
    my $Ln = length($n);
    return $n if $Ln <= 3;

    while($c <= $Ln) {
        substr($n, - $c, 0, ','); # insert
        $c += 4;
    }

    return $n;
}


print HUMANo($_) for qw[ 1 11 111 1111 11111 111111 1111111 ];

__END__
1
11
111
1,111
11,111
111,111
1,111,111