Here is some working code, based on yours Plus this msdn article.

#!/usr/bin/perl -w
use strict;
use Win32::OLE('in');
use Data::Dumper;

use constant wbemFlagReturnImmediately => 0x10;
use constant wbemFlagForwardOnly => 0x20;

my @computers = qw( WWMONITOR3 );
foreach my $computer (@computers) {
  
   my $objWMIService = Win32::OLE->GetObject("winmgmts:\\\\$computer\\
+root\\CIMV2") or die "WMI connection failed.\n";
   my $colItems = $objWMIService->ExecQuery( 
                  "select FreeSpace,Size,Name from Win32_LogicalDisk w
+here DriveType=3",
                   "WQL",
                  wbemFlagReturnImmediately | wbemFlagForwardOnly);

   foreach my $Disk( in $colItems){
      print  "$computer : Drive  $Disk->{Name} has " .
             format_kilo($Disk->{FreeSpace}) . " free (" .
             format_kilo($Disk->{Size}) . " total)\n";
   }
}

###############################################################
sub format_kilo  # Kilo, mega and gig
{
    my $number = shift;
    my $fixwidth = shift;
    my $suffix = " ";

    if ($number > 0x40000000)
    {
        $number /= 0x40000000;
        $suffix = 'G';
    }
    elsif ($number > 0x100000)
    {
        $number /= 0x100000;
        $suffix = 'M';
    }
    elsif ($number > 0x400)
    {
        $number /= 0x400;
        $suffix = 'K';
    }

    # Split integer and decimal parts of the number and add commas
    my $integer = int($number);

    # Add Leading spaces if fixed width
    $fixwidth and $integer =  ' ' x ($fixwidth - length($integer) - le
+ngth($suffix)) . $integer;

    # Combine it all back together and return it.
    return $integer.$suffix;
}
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UPDATE:
Your code would work if you checked the value of $objItem->{DriveType} , and printed only when that had a value of "3" (Logical Disk). Most likely, your last disk has a value of "5" (CD ROM), which does not return valid FreeSpace or other attributes, uless it has a CD mounted.

Hence, if you want to look at VOLUMES, make sure you are not looking at mapped drives, CD's etc - and limit your examination to {DriveType} == 3.