Both structured design and the newer object-oriented design discourage using subroutines that share a common pool of variables

Structured code does not preclude shared variables.

And the basis of OOP is the association of attributes (variables) with methods (subroutines). It's literally built around a common pool of variables.

However, as HaukeX suggested, it might be time to learn to write OO Perl. :-)

I think it's fair to say that, in object-oriented design, the fields/attributes/properties/member variables of a class are a common pool of variables shared by subroutines/methods.

I'd look at it differently - while of course the subs are part of the class and technically all objects of the class share them, each object is a separate entity, so I'd look at it as each object having its own copy of the instance variables - or in Perl, really just one variable, typically referred to as $self. I would not expect to see any variables declared outside of the subs of the class, perhaps the only exception being default settings like I showed here.

(The concept of Inside Out objects also exists, but I practically never see that in the wild and personally wouldn't suggest it.)

I see what you mean. Now that I have read a bit about OO Perl I see that what I've been creating would be considered "class variables" (a.k.a. static member variables) in C++, which is the extent of my OO experience. As you point out, they have their place, but they're not the same as instance variables.

I have to disagree in this generality, closures are a common technique.

I have to disagree in this generality, closures are a common technique.

I'm confused by this comment, could you show a code example of what you mean?

My understanding of BillKSmith's node is that it is arguing against file-scoped lexicals that are used by multiple subs in the file, and I fully agree with that.

One contrived example, using a private config-val instead of a public package var.

my $config = 42;
my $change_allowed = 0;

sub mult {
  $_[0] * $config 
}

sub get_config {
   $config
}

sub set_config {
   $config = shift if  $change_allowed;
}

# yadda
[download]

of course you could also put only those subs inside an extra block together with the closure var.

Cheers Rolf
_{(addicted to the 𐍀𐌴𐍂𐌻 Programming Language :)
Wikisyntax for the Monastery}