use strict;
use warnings;

my $lookup;
while(my($parent, @children) = <DATA> =~ /(\S+)\s*/g) {
  $lookup->{$parent} ||= [];
  push @{$lookup->{$parent}}, @children;
}

sub build {
  my $children = delete $lookup->{+shift} || [];
  my $parent = {};
  $parent->{$_} = build($_) for(@$children);
  return $parent;
}

my $tree = build('ROOT');

use Data::Dumper;
print Dumper $tree;

__DATA__
ROOT a b c
b e f
a d