use strict;
use warnings;

use Data::Dumper ();

my %shash;
my $line;

while (defined($line = <DATA>)) {
   # Get rid of end of line.
   chomp($line);

   # Build hash from line.
   my $p = undef;
   $p = { $_ => $p } foreach (reverse(split(/--/, $line)));

   # Merge hashes.
   my $key;
   my $base = \%shash;
   for (;;) {
      ($key, $p) = each(%$p);
      last unless ($base->{$key});
      $base = $base->{$key};
   }
   $base->{$key} = $p;
}


print(Data::Dumper::Dumper(\%shash));

__DATA__
Item1--Item2--Item3
ItemX--Item2--ItemA
Item1--ItemV--Item3--Item4