About freezes:

foreach my $done ( 1 .. 100 ) { FS_sweep( 'C:', \&FS_file_big ); ...
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Running this script several times in Emacs, using the compile command, all resulted in a freeze. The the number of calls to FS_sweep before freeze was between 0 and 91.

Running the script in the command shell resulted in 0 to 14 succeeded calls to FS_sweep. In most cases none.

… <C:/Users/bo/Application Data/À> … (was this a question/problem?) …

use strict;
use warnings;
use 5.010;

use Data::Dump qw(dump dd ddx);

use Win32::LongPath;
use Fcntl ':mode';

sub do_readdirL_arr {
    my $dir_path = shift;

    my $dir = Win32::LongPath->new;
    unless ( $dir->opendirL($dir_path) ) {
        warn "!! unable to open $dir_path ($^E)";
        return;
    }
    my @name = $dir->readdirL();
    return \@name;
}

sub do_readdirL_while {
    my $dir_path = shift;

    my $dir = Win32::LongPath->new;
    unless ( $dir->opendirL($dir_path) ) {
        warn "!! unable to open $dir_path ($^E)";
        return;
    }
    my @name;

    while ( my $name = $dir->readdirL() ) {
        push @name, $name;
    }
    return \@name;
}

my @path = (
    'C:/Documents and Settings',          'C:/ProgramData/Application 
+Data',
    'C:/ProgramData/Desktop',             'C:/ProgramData/Start Menu',
    'C:/ProgramData/Templates',           #'C:/Users',
    'C:/Windows/appcompat/Programs',      'C:/Windows/System32/Com/dmp
+',
    #'C:/Windows/System32/spool',
    'C:/Windows/System32/Tasks',
    'C:/Windows/System32/Tasks_Migrated', 'C:/Windows/SysWOW64/Com/dmp
+',
    'C:/Windows/SysWOW64/Tasks',          'C:/Windows/Temp',
);

foreach my $path (@path) {
    say "$path: array: ", dump( do_readdirL_arr($path) ), ' while: ',
      dump do_readdirL_while($path);
}

__DATA__
C:/Documents and Settings: array: ["g"] while: ["~"]
C:/ProgramData/Application Data: array: ["u"] while: ["{"]
C:/ProgramData/Desktop: array: ["H"] while: ["|"]
C:/ProgramData/Start Menu: array: ["~"] while: ["m"]
C:/ProgramData/Templates: array: ["K"] while: ["e"]
C:/Windows/appcompat/Programs: array: ["u"] while: ["Q"]
C:/Windows/System32/Com/dmp: array: ["m"] while: ["l"]
C:/Windows/System32/Tasks: array: ["\\"] while: ["\x7F"]
C:/Windows/System32/Tasks_Migrated: array: ["\22"] while: ["\f"]
C:/Windows/SysWOW64/Com/dmp: array: ["p"] while: ["u"]
C:/Windows/SysWOW64/Tasks: array: ["["] while: ["p"]
C:/Windows/Temp: array: ["\x{105}"] while: ["\x{110}"]
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__DATA__ contains an example of output. It changes for every run.

I have no access to the directories using the MS File explorer.

When I in FS_sweep skip those directories, 'C:/Users' and C:/Windows/System32/spool' there are no freezes.

Now it's getting sad/worrisome/interesting. With my old 5.26 Perl, your script above reports empty arrays as expected (and, as a side note, some reasonable content if run with administrative rights). Also, I finally did mega-stress-test (sweep C:/ for 100 times), it completes OK.

Then I tried latest recommended Strawberry Perl versions 5.30.2.1, 5.28.2.1, 5.26.3.1, -- under 5.28 and 5.30 results are as you report: random bytes/single characters for "protected operating system" (as MS UI calls them) folders. The "mega-test" (tried once) hung on 5th sweep, though I was able to ctrl-C it. So, definitely looks like a bug (somewhere in Win32::LongPath XS guts?).

Update. Actually, the bug is not related to "special system" directories, and I'm no longer sure Win32::LongPath under 5.26 behaved correctly, too. Maybe the call to opendirL shouldn't succeed in the first place.

>perl -v

This is perl 5, version 30, subversion 2 (v5.30.2) built for MSWin32-x
+64-multi-thread

Copyright 1987-2020, Larry Wall

Perl may be copied only under the terms of either the Artistic License
+ or the
GNU General Public License, which may be found in the Perl 5 source ki
+t.

Complete documentation for Perl, including FAQ lists, should be found 
+on
this system using "man perl" or "perldoc perl".  If you have access to
+ the
Internet, point your browser at http://www.perl.org/, the Perl Home Pa
+ge.


>mkdir abc

>perl -MData::Dump -MWin32::LongPath -E "for (1..5) {my $lp=Win32::Lon
+gPath->new; $lp->opendirL('abc') or die; dd [$lp->readdirL]}"
[".", ".."]
[".", ".."]
[".", ".."]
[".", ".."]
[".", ".."]

>icacls abc /deny Everyone:F
processed file: abc
Successfully processed 1 files; Failed processing 0 files

>perl -MData::Dump -MWin32::LongPath -E "for (1..5) {my $lp=Win32::Lon
+gPath->new; $lp->opendirL('abc') or die; dd [$lp->readdirL]}"
["\b"]
["\b"]
["\35"]
["\32"]
["'"]

>perl -MData::Dump -MWin32::LongPath -E "for (1..5) {my $lp=Win32::Lon
+gPath->new; $lp->opendirL('abc') or die; dd [$lp->readdirL]}"
["\3"]
["\26"]
["#"]
["\22"]
[4]

>perl -MData::Dump -MWin32::LongPath -E "for (1..5) {my $lp=Win32::Lon
+gPath->new; $lp->opendirL('abc') or die; dd [$lp->readdirL]}"
["M"]
["~"]
["M"]
["w"]
["I"]

>perl -MData::Dump -MWin32::LongPath -E "for (1..5) {my $lp=Win32::Lon
+gPath->new; $lp->opendirL('abc') or die; dd [$lp->readdirL]}"
["M"]
["}"]
["C"]
["l"]
["}"]

>icacls abc /remove:d Everyone
processed file: abc
Successfully processed 1 files; Failed processing 0 files

>perl -MData::Dump -MWin32::LongPath -E "for (1..5) {my $lp=Win32::Lon
+gPath->new; $lp->opendirL('abc') or die; dd [$lp->readdirL]}"
[".", ".."]
[".", ".."]
[".", ".."]
[".", ".."]
[".", ".."]

>rmdir abc

>
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Thanks vr for the help!

I am using Strawberry Perl (64-bit) 5.30.1.1-64bit.

"Freeze" sounds like you're running out of RAM , https://en.wikipedia.org/wiki/Paging#Swap_death is bad mmkay ;)

The solution is to dump before you freeze