#!/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