I don't know of any existing routine, but I think something like this should work:

use Net::Netmask;

my @blocks = (...); # list of your cidr blocks
my @sorted = sort { ($a->bits <=> $b->bits) || ($a->base <=> $b->base)
+ } @blocks;
my @children;
my @parent;
for my $i (0..$#sorted) {
  for (my $j = $i-1; $j >= 0; $j--) {
      if ($sorted[$j]->contains($sorted[$i])) {
          $parent[$i] = $j; # or = $sorted[$j];
          push(@{$children[$j]}, $i); # or $sorted[$i]
          last;
      }
  }
}
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If you want the whole tree rooted, just add:

unshift(@sorted, new Net::Netmask("0.0.0.0/0")); # one subnet to rule 
+them all
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