Another solution is using (abusing?) list auto-stringification, or multidimensional emulation. (How long should the term be for something so simple?) Instead of all those cool hash-reference trees, you could use a flat hash with namespaced keys, just like your file. If you feed perl a list for a hash key, it will apply a join('', ...) to it. (I believe there is a perlvar to change the separator, though. <looks> Oh, It's $;) After you build the hash this way, walking your tree is is as simple as sort keys.

$; = ':';

my %Hash;

while (<>) {
   chomp $_;
   my ($key, $val) = split(/=/);
   if ($key =~ /:/) {
      my @keyparts = split(/$;/, $key);
      $Hash{ join("$;", @keyparts) } = $val;
        # I'm not sure how to get perl to not scalar-ize it in this ca
+se
        # But it's probably better to reduce cargo-cultism ;)
   } else {
      $Hash{$key} = $val;
   }
}

foreach (sort keys %Hash) {
   print "$_    \t=> " . $Hash{$_} . "\n";
}

print $Hash{'a1','b1','c1'} . "\n"
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