One, perhaps final, comment. In the process of writing a short program to reproduce the behavior (in case someone, probably me, decides to file a bug report, my earlier whinging notwithstanding), I came up with this:

#!/usr/bin/perl -w

use strict;
use File::Glob ':glob';

my $dir = shift || "/etc";
my $max = shift || 10000;
my $entries = 0;
while (glob("$dir/*")) {
    last if (++$entries > $max);
}
print($entries, " items under $dir\n");
[download]

If I comment out the use File::Glob ':glob'; to get a run with the core glob, then put it back in, I see the following

  time globbug.pl /etc 20000
279 items under /etc

real    0m0.00s
user    0m0.00s
sys    0m0.00s

  time globbug.pl /etc 280  
281 items under /etc

real    0m0.15s
user    0m0.09s
sys    0m0.06s
[download]

The first (core) run demonstrates that there are 279 entries in my /etc, and it takes almost no time to determine that. The second run, confined to stop after it is demonstrably wrong, takes quite a while to be wrong. This is consistent with it rebuilding the entire list for each iteration, then throwing away all but the last item.