note nothingmuch Anonymous Monk's code can be slightly beautified: <c> 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 structure # 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 delete it if it's there delete $deep{$child}; }; } </c> <div class="pmsig"><div class="pmsig-205152"> -nuffin<br>zz zZ Z Z #!perl </div></div> 538326 538326