If you have control over the file format, i siuggest you change it:
<br

1. Don't use whitespace as a delimiter. This is a broken concept. It will csues endless probelms when you copy-and-paste, change editors, etc. (If you REALLY like syntactically significant whitespace, you should be using Python.)
2. Don't repeat each cate/subcat in the contained data. This is error-prone, redundant, and makes restructuring dificult.

Instead, why not XML?

#!/usr/bin/perl -w

use XML::Simple;
use Data::Dumper;


$xml =
' <file>
<cat1>
    <sub1>
        <sub-sub1/>
    </sub1>
</cat1>
<cat2/>
<cat3>
    <sub1/>
</cat3>
</file>
';
my $c = XMLin($xml);
# substitute filename for $xml here

print Dumper($c);
__END__
[download]

Results:

$VAR1 = {
          'cat1' => {
                      'sub1' => {
                                  'sub-sub1' => {}
                                }
                    },
          'cat2' => {},
          'cat3' => {
                      'sub1' => {}
                    }
        };
[download]

Bob Niederman, http://bob-n.com