package Y1;

sub new { 
  ...
  $self->{X1} = X1->new();
  $self->{MANY_X2} = [ X2->new(), X2->new() ];
  ...
}

package main;

my $obj = Y1->new();
print $obj->{X1}->{X1prop};
print $obj->{MANY_X2}->[1]->{X2prop};