You really don't need Tree::Simple to do that. A simple approach could be:

• Read the file in a hash in the form child => parent
• Get all the leaves: keys of the hash that are not used also as values (keys, grep)
• For each leaf
  • Generate an array with its "genealogy" searchin in the hash till you get a value that's not a key (a root)
  • reverse the array
  • join
  • print

All considered it should be no more than 20 lines of code.

Update: make it 22...

#!/usr/bin/perl 
use strict;
use warnings;

open my $f,"<","x.txt";
my %tree;
while (<$f>) {
  chomp;
  my ($c,$p)=split(/:/,$_);
  $tree{$c}=$p;
}
my %eert = reverse %tree;
foreach my $l (keys %tree) {
  if (!defined($eert{$l})) {
    my @g=($l);
    while($l) {
      $l=$tree{$l};
      push(@g,$l) if defined($l);
    }
    print join(':',@g),"\n";
  }
}
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