How can I catch access to member variables?
$Class1->{Class2}
If the Class2 field doesn't exist, is is possible to catch this from an internal function?
You do this by providing proper getter/setter methods that wrap around your class/instance variables. The internals should never be accessed directly, particularly from outside of the class itself (it's wise to not do so within the class either, except for the actual method that maintains that specific attribute. Here's a very basic example:
use warnings;
use strict;
package A;
sub new {
my ($class, %args) = @_;
my $self = bless {}, $class;
$self->x($args{x});
$self->y($args{y});
return $self;
}
sub x {
my ($self, $x) = @_;
$self->{x} = $x if defined $x;
return $self->{x} // 1;
}
sub y {
my ($self, $y) = @_;
$self->{y} = $y if defined $y;
return $self->{y} // 2;
}
package main;
my $obj = A->new(x => 5, y => 3);
print $obj->x ."\n";
print $obj->y ."\n";
Now, you could just as easily do print $obj->{x}, but that's where your problem is. What happens when the code is much more complicated than this, and for some reason you want to change the x attribute name to foo, but retain the x() method? $obj->{x} will now be undef as its never set.
Always use the provided methods for accessing attributes of a class/object. Encapsulation such as this is a staple of OO programming.