use strict;
use warnings;

use Data::Dumper qw( Dumper );

my %tree;

while (<DATA>) {
   chomp;
   my $p = \%tree;
   foreach (split(qr{\s*/\s*}, $_)) {
      $p = $p->{$_} ||= {};
   }
}

print Dumper \%tree;

__DATA__
one
foo / bar
foo / baz
foo / qux / two
foo / qux / three
foo / qux / four
five