Okay thanks arden and eserte for you help. Here is the solution:

#!perl
use Tk;
use Tk::BrowseEntry;
use Win32::DriveInfo;
@drives = Win32::DriveInfo::DrivesInUse();
@drives = map ($_ . ":", @drives);
@freelet = Win32::DriveInfo::FreeDriveLetters();
@freelet = map ($_ . ":", @freelet);

$mw = new MainWindow;
$d = $mw->BrowseEntry(-label => "Mapped Drives", 
            -variable => \$var,
            -state => 'readonly',
            -choices => \@drives)->pack;
$f = $mw->BrowseEntry(-label => "Free Drives",
            -variable => \$var1,
            -state => 'readonly',
            -choices => \@freelet)->pack;
MainLoop;
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