my $filename = join('', split('\.', $ENV{REMOTE_ADDR}) );
...
# then, somewhere write to / read from the file

##</code><code>##

my @numbers = split('\.', $ENV{REMOTE_ADDR});
if( ! -d $numbers[0] ) {
    mkdir($numbers[0]);
}
if( ! -d "$numbers[0]/$numbers[1]" ) {
    mkdir("$numbers[0]/$numbers[1]");
}

my $path = "$numbers[0]/$numbers[1]";
my $filename = "$path/" . join('', @numbers);