Perhaps Recursively walk a hash to get to an element would serve as a good starting point.

Update: I'm sure it could be done, but probably tougher than necessary. So here's a solution from scratch:

use strict;
use warnings;

my @d; # the data structure we're building.

my @p; # current (actually previous) vector

while (<DATA>)
{
    chomp;
    my $r = \@d;
    my @vc = my @v = split m# / #;
    my @pc = @p; # bashable copy
    while ( @vc and @pc and $vc[0] eq $pc[0] )
    {
        shift @vc; shift @pc;
        $r = $r->[-1]{'children'};
    }
    while ( @vc )
    {
        push @$r, { name => shift @vc };
        $r = ( @vc and $r->[-1]{'children'} = [] );
    }
    @p = @v;
}

use Data::Dumper;
print Dumper \@d;

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

Note, this presumes that the input lines are in proper order, as they are in your example data.