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.