#perl -e;
use Data::Dumper;
my %nodes;
my @children;

while (<DATA>) {
    chomp;
    my ($c,$p)=split/:/,$_;
    push @children,$c unless $nodes{$c};
    $nodes{$_}{name}||=$_
        for $c,$p;
    $nodes{$p}{kids}{$c}=$nodes{$c};
}
delete $nodes{$_}
    for @children;
my @roots=keys %nodes;

print Dumper($nodes{$_}) for @roots;


__END__
b:a
c:a
d:b
e:c
f:c

##</code><code>##

$VAR1 = {
          'name' => 'a',
          'kids' => {
                      'c' => {
                               'name' => 'c',
                               'kids' => {
                                           'e' => {
                                                    'name' => 'e'
                                                  },
                                           'f' => {
                                                    'name' => 'f'
                                                  }
                                         }
                             },
                      'b' => {
                               'name' => 'b',
                               'kids' => {
                                           'd' => {
                                                    'name' => 'd'
                                                  }
                                         }
                             }
                    }
        };