#!/usr/bin/perl 
use strict;
use warnings;

my $start = '2A';
my %data;

for my $line (<DATA>) {
	chomp $line;
	
	my ($key, $value) = split / --> /, $line;
	
	if ( exists $data{$key} ) {
		push @{ $data{$key} }, $value;
	} else {
		$data{$key} = [$value];
	}	
}

do_tree( $start );


sub do_tree {
	my $start = shift;
	
	for my $node (@{ $data{ $start } }) {
		print "$start --> $node\n";
		do_tree( $node );
	}	
}

__DATA__
1A --> 2A
1A --> 2B
2A --> 4A
2A --> 5A
4A --> 5A
2B --> 3A
3A --> 4A