sub new {                  # @_  = ('Music', 5)
    my $class = shift;     # @_  = (5)
    my $number1 = @_;      # $number1 = 1  - array in scalar context r
+eturns it's size! 
    my $number2 = &add_numbers; #  here you call sub add_numbers!
[download]

&add_numbers call the sub with current argiment list @_. You already shifted object reference from it, so it contains only 5.

Then

sub add_numbers {         # @_    = (5)
    my $self = shift;     # $self =  5 !!!
    my $value = $self->{"Number_one"} ;  # ERROR!
[download]

see perlsub