The name keys seem redundant to me. If you want to generate a nested hash structure based on the key strings then the following (non-recursive) code may get you started:

use strict;
use warnings;
use Data::Dump::Streamer;

my %hash;

while (<DATA>) {
    chomp;
    my @elements = split /\s*\/\s*/;
    
    next if ! @elements;
    
    $hash{$elements[0]} = {} if ! exists $hash{$elements[0]};
    my $subHash = $hash{shift @elements};
    
    for (@elements) {
        $subHash->{$_} = {} unless exists $subHash->{$_};
        $subHash = $subHash->{$_};
    };
}

Dump \%hash;

__DATA__
one
foo / bar
foo / baz
foo / qux / two
foo / qux / three
foo / qux / four
five
[download]

Prints:

$HASH1 = {
           five => {},
           foo  => {
                     bar => {},
                     baz => {},
                     qux => {
                              four  => {},
                              three => {},
                              two   => {}
                            }
                   },
           one  => {}
         };
[download]