I thought I had such a program in the DBD::WMI distribution, but I didn't. I've added the following example to (the next release of) DBD::WMI, and it works and tells me the added/removed drive letter:

#!/usr/bin/perl -w
package main;
use strict;
use DBI;
use Data::Dumper;

my $dbh = DBI->connect('dbi:WMI:');

my $sth = $dbh->prepare(<<WQL);
    SELECT * FROM __instanceoperationevent
    WITHIN 5
    WHERE TargetInstance ISA 'Win32_LogicalDisk'
    and TargetInstance.Name != 'C:'
WQL
$sth->execute();

# loops forever
while (defined (my $row = $sth->fetchrow_arrayref())) {
    my $ev = $row->[0];
    
    # Windows XP only :(
    print "Class\t",$ev->Path_->Class,"\n";
    for (qw(Name DeviceID Caption MediaType InterfaceType)) {
        print join "\t", $_, ($ev->TargetInstance->{$_} || '<none>');
        print "\n";
        if ($ev->Path_->Class eq '__InstanceCreationEvent') {
            print "Device (letter) $ev->{TargetInstance}->{Name} added
+\n";
        }
    }
}
[download]

The restriction on .InterfaceType to "USB" does not work, because the Win32_LogicalDisk does not have that property, unfortunately. So you'll have to hope that the added drive letter actually corresponds to the/a USB drive.