use strict;
use warnings;
use YAML qw/Dump/;

my %parent_of;
my %tree;
my %ref;

while(<DATA>) {
    chomp;
    my ($parent, $child) = split /,/;
    $parent_of{$child} = $parent;
    my $h;
    if(exists $parent_of{$parent}) {
        $h = $ref{ $parent };
        $h->{$child} = {};
        $ref{$child} = $h->{$child};
    } 
    else {
        $h = \%tree;
        $ref{$parent} = $h;
        $h->{$parent}{$child} = {};
        $ref{$child} = $h->{$parent}{$child};
    } 
} 
print Dump(\%tree) . "\n"; 

__DATA__
1,3
1,4
1,5
3,8
3,9
4,11
4,18
8,14
8,15
5,17
5,16
17,21