since references aren't your strongest point, try explicitly assigning variables to them. it really helps to get a feel of what's going on...

sub print_tree
{
    my $key = shift;
    my $tab = shift;
    
    print "$tab$key\n";
    my $array_ref = $found{$key};
    if ($array_ref)
    {
        $tab .= "\t";
        foreach my $element (@{$array_ref})
        {
            print_tree($element,$tab);
        }
    }
}

__DATA__

output:

start
    a
        c
            f
        e
    b
    d
        e
    g
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