Having run both the OPs version and yours they both come back with 0.

use warnings;
use strict;
use Mail::Webmail::Gmail;

my $gmail = Mail::Webmail::Gmail->new( 
                username => '', password => '', proxy_name => 'localho
+st:3128'
            );

my @labels = $gmail->get_labels();

print "Number of labels returned: ".scalar (@labels)."\n";
    
foreach my $label (@labels) {
    print "Label: $label\n";
}
[download]

From my test above, it would appear that Mail::Webmail::Gmail->new() completes without error even with a known bad password.

2011-01-19 09:29 GMT - restored (in part) after discussion with ww