Well, require is for loading files, which you won't need at all if everything is inline, so removing it is a good idea.

The reason I would prefer 'use base' (bart's objections notwithstanding) over assigning directly to @ISA is that I don't like having run-time code in modules. (It also seems to work fine in a test here - just remove the @Class2::ISA line and use base qw/Class2/; instead of require Class2;).

I would say that having a script with a single entry point and execution following function/method invocation, it is easier to see what is going on. Otherwise one must read through the whole file to check for side-effects.

For example, I commonly have a

main</c< function:
<code>
#!/usr/bin/perl
use warnings;
use strict;

exit !main(@ARGV);

sub main {
  ...
}

sub foo {
}
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The benefit of the early exit is that a maintainer can see that there isn't any run-time code lurking down between any later subs (since execution never flows down there). It also means that any lexical variables I use in main aren't accidentally accessible in the functions within the script. (OK, it might make sense to have some file-scope vars, but such things should stand out, not be the default because I'm writing code at the toplevel).

Yes, for quick scripts etc it is handy to have the scripting behaviour, but by the time we bring OO or helper functions in, we probably have enough complexity that this kind of discipline is beneficial.

So...when using perl as a programming language (as opposed to a 'scripting' language), I think it is harmful to have code at the top-level - it's effects are too wide-spread (variable scope too large) and the code can be spread over too large an area to see easily.