I suggest you copy and paste the code I gave earlier. It is not what you implied I suggested. Consider:

use strict;
use warnings;

my $data_dir = 'c:\\Wibble';
my $data = 'Plonk';
my $data_brf = "$data_dir\\$data\.brf";

print "$data_brf\n";
[download]

Prints:

c:\Wibble\Plonk.brf
[download]

If that is not what you want you better show us the contents of $data_dir and $data, and show us what you expect to see printed.