You're testing the files and directories as if they were in your current working directory, not the one you're examining. Here's a version that would work:

#!/usr/bin/perl
use strict;
use warnings;

my $dir = "D:/test_dir";
opendir( DIR, $dir);
my @file_list = readdir(DIR);
closedir(DIR);


print "Decision whether it is a file or a directory:\n";
foreach ( @file_list )
{
    if( (-d "$dir/$_") ) 
    {
        print "directory: " . $_ . "\n";
    }

    if( (-f "$dir/$_") )
    {
        print "file: " . $_ . "\n";
    }
}

print "\n\n";

print "Just print everything:\n";
foreach ( @file_list )
{
    print $_ . "\n";
}
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