use strict;
use Data::Dumper;
my %deep;

{
  my %flat;

  while (<DATA>) {
    chomp;

    my ( $child, $parent ) = /^(.*?):(.*?)$/;

    # if the parent is new then it's possibly at the root of the struc
+ture
    # like 'a' is
    unless ( exists $flat{$parent} ) {
        $flat{parent} = $deep{parent} = {};
    }

    # find the parent, and mark this node as a child of it
    $flat{$parent}{$child} = ($flat{child} ||= {})

    # since $child is a child of $parent it can't be at the root, so d
+elete it if it's there
    delete $deep{$child};
  };
}
[download]