Actually, I felt like writing a little homework today :-) I leave the rest of prettyprinting to you.

use warnings;
use strict;

my (%edges, %processed);

while (<DATA>) {
        if (/(\d+),(\d+)/) {
                $edges{$1} = [] unless (exists $edges{$1});
                push(@{$edges{$1}}, $2);
        }
};
close DATA;

for (sort keys %edges) {
        print_tree($_) unless exists $processed{$_};
}

sub print_tree {
        my ($node, $pp) = @_;
        $pp = '' unless defined $pp;
        print "$pp$node\n";
        $processed{$node} = 1;
        return unless exists $edges{$node};
        for (@{$edges{$node}}) {
                print "$pp$_ - cycle detected\n" if exists $processed{
+$_};
                print_tree($_, $pp.' ') unless exists $processed{$_};
        }
}
__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
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