a global filehandle is not necessary - a lexical declared in the module will do exactly the same job

My original argument was (intended to be) that global file handles and file-scope lexical file handles are equivalent, and the choice between them is a neutral matter of style.

this seems to be an argument for lexical filehandles rather than global ones

This comes back to a matter of style — for some programmers it may be such an argument, for others ALLCAPS tokens may be in a different "five, plus or minus two" from regular variables.

I think the usage of globals can be avoided 99.9% of the time

I agree that the use of globals generally should be minimized, but I consider file-scope lexicals (especially in the main script) a "sneaky" form of global variable. My concern is opposing unthinking "never use globals!" policies that then result in using file-scope lexicals in exactly the same way as globals — except that lexicals are harder to inspect.

On a side note, a file-scope lexical can possibly have wider scope than a global if the same file defines multiple packages. Each global is contained in its package, but the lexical will persist across package statements to the end of the file.

I disagree with it being harder to debug; issues arising from incorrectly used globals are IMHO much more annoying to debug

File-scope lexicals effectively are globals, except that they are harder to inspect in the debugger. The "even harder to debug" remark was in reference to a "never use globals!" approach where the callback is wrapped in a closure that also carries references to lexicals in the block that will use it. The effect is global variables that are not actually in the symbol table, and (as far as I know) are almost impossible to debug because there is no way to look inside a closure.

My original argument was (intended to be) that global file handles and file-scope lexical file handles are equivalent, and the choice between them is a neutral matter of style.

But they're not equivalent, I've already named two disadvantages (no typo protection and potential clashes with other globals like package names), and I have yet to hear an advantage to bareword filehandles.

Even in the limited case that you name (file-scope lexical file handles), there is yet another difference: if there ever is a clash in names, with lexicals it's incredibly easy to limit the scope of the issue: simply place the statements in a block, and you've limited the scope of the lexical, including a visual scope that allows one to glance at the code and know with certainty that this filehandle is contained within that scope. If one were using bareword filehandles instead, their scope is the entire package, beyond the bounaries of any blocks, so either you'd have to go through the entire package, renaming the filehandles to eliminate any name clashes, or you'd have to use bare blocks and the local *FH "hack", which has its own disadvantages. I see this as yet another disadvantage to bareword filehandles, again with no advantage.

... a file-scope lexical can possibly have wider scope than a global if the same file defines multiple packages ...

That's true, but again easily solved by placing the package in a block, and Perl 5.14 introduced the package NAMESPACE BLOCK syntax to make this look even nicer. Some people even argue against multiple packages in one file. And even with the (admittedly sometimes confusing) issue of lexicals potentially crossing package boundaries, the scope of the lexical is still "visually" limited to the file; I would argue that name clashes because a global of the same name was used in a different file is a much more tricky issue.

My concern is opposing unthinking "never use globals!" policies that then result in using file-scope lexicals in exactly the same way as globals ...

Yes, a valid concern, but like you, I would argue that file-scope lexicals used in exactly the same way as globals are globals too. But that kind of dogmatic policy is not what I meant (or said). Instead, IMHO "globals can and should nearly always be avoided" is intended to cause people to think about what would be a better solution, which is usually a change in design.

To someone who knows what they're doing, it may be acceptable to sometimes use globals, but again, this thread is in the context of giving advice to a beginner.

... (as far as I know) are almost impossible to debug because there is no way to look inside a closure.

I'm far from an expert with the debugger, but that doesn't appear to be correct.

$ cat x.pl 
use warnings;
use strict;
sub x {
    my $y = 123;
    sub {
        $y += shift;
        print "$y\n";
        print shift->(), "\n";
    }
}
my $z = x;
my $foo = "abc";
$DB::single=1;
$z->(111, sub {
    return $foo."def";
});

$ perl -d x.pl 
...
main::(x.pl:11):    my $z = x;
  DB<1> c
main::(x.pl:16):    });
  DB<1> s
main::CODE(0x543ed97f20a0)(x.pl:6):
6:            $y += shift;
  DB<1> y 0
$y = 123
  DB<2> s
main::CODE(0x543ed97f20a0)(x.pl:7):
7:            print "$y\n";
  DB<2> y 0
$y = 234
  DB<3> s
234
main::CODE(0x543ed97f20a0)(x.pl:8):
8:            print shift->(), "\n";
  DB<3> s
main::CODE(0x55d4d6a31da8)(x.pl:15):
15:        return $foo."def";
  DB<3> y 0
$foo = 'abc'
$z = CODE(0x543ed97f20a0)
   -> &main::__ANON__[x.pl:9] in x.pl:5-9
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no typo protection

This can be a legitimate concern in that you will get only a run-time error instead of a compiler error, but I have yet to write code that had enough bareword file handles for this to be a problem for me.

potential clashes with other globals like package names

Is this the origin of the convention in Perl of writing file handle names in ALL UPPERCASE? I had picked that up without really knowing why, but preventing that clash (who tries to name a file handle UNIVERSAL?) seems like a good reason for the convention.

solved by placing the package in a block

Which I do in my code:

package Outer;
# ...
{
  package Outer::Inner;
  # ...
}
[download]

I also limit this to OOP code where Outer::Inner is an internal implementation detail of Outer. Using multi-package files too much is a good way to go insane, even with Emacs' CPerl mode and speedbar for navigation.

think about what would be a better solution, which is usually a change in design

I agree with this in the case of a proliferation of file handles, but bareword file handles remain a useful tool in producing concise I/O code in a main script, while also being visually distinctive in their own way, with both ALLCAPS and different syntax highlighting from variables. This is probably why I have not had typo problems with them, now that I notice it.

this thread is in the context of giving advice to a beginner

The concern I have is the possibility of advice that works well for a beginner, but could unintentionally limit their future growth. I am unsure exactly how "always use lexical file handles" would do that, but I am suspicious of "always" and "never" in general.

almost impossible to debug because there is no way to look inside a closure

that doesn't appear to be correct

Yes, you can examine locals if you have execution stopped inside the closure, but given a CODE ref in a structure somewhere, how do you get the closed over values out of it?