Generally, if you're not affraid of 'outside', you can use proc fs popular in many unices:

opendir(D,"/proc/$$/fd/");
while ($fd=readdir(D)) {
print "I've got this descriptor opened: $fd";
};
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and then, you could use BrowserUK's POSIX::close to close it;

But this is not very portable, works on linux with procfs mounted, as soon as you move to win32, you're in trouble.

I wanted something working from inside