The tree I am constructing is based on code references calling the parents subs for its parents and relations:

use strict;
use warnings;

my %data;
while(<DATA>){
  my( $child, $parent, $relation ) = split /,\s*/;
  $data{$child} = { parent => sub { $child.(exists $data{$parent}?'<-'
+.$data{$parent}{parent}->():"<-$parent") }, 
                    relation => sub { (exists $data{$parent}?$data{$pa
+rent}{relation}->().'.':'' ).$relation } };
}

print $_->{parent}->()." = ".$_->{relation}->()."\n" for values %data;

__DATA__
M10,    Q,    P,
M143,    M10,    P,
M406,    M143,    PL,
M407,    M143,    PL,
M420,    M143,    E,
M421,    M143,    E,
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