First, a couple of questions:

• How accurate do you need to be? (this method can give a slightly different number for free space than right-clicking and selecting properties)
• Can you map a drive to the share before checking, and disconnect after?
• Is Windows Script Host installed?

If the answer to these questions are "99% is good enough","yes" and "yes", try this:

#!perl

use warnings;
use strict;
use Win32::OLE;

my $fs = Win32::OLE->CreateObject('Scripting.FileSystemObject');
my $d = $fs->GetDrive('g:');
print $d->{TotalSize}, " total\n";
print $d->{FreeSpace}, " free\n";
print $d->{AvailableSpace}, " available\n";
print $d->{TotalSize} - $d->{FreeSpace}, " used\n";
__END__
Output on my system, with G: mapped to a Samba share:
19447939072 total
16677601280 free
16677601280 available
2770337792 used
[download]

where AvailableSpace is the space available to you and FreeSpace is total free space.

Note - this code requires you already have the drive mapped before you try it.

Update: added closing </ul> tag

Update 2: Please read BrowserUK's caveat below!