(@dir0) = readdir(ID) || print "Couldn't read directory contents.\n";
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|| forces scalar context. Your code actually is:

@dir0 = (readdir(ID) || 1); # assuming print doesn't fail
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What you probably want, is:

(@dir0 = readdir(ID)) || print "...";
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which is equal to:

@dir0 = readdir(ID) or print "...";
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(|| and or are not equal: || is tighter (see perlop).)

Update (20020303021949)
In my chatterbox nodelet, I read: crazyinsomniac says readdir fails? mention of -MO=Deparse couldn't hurt ;)
So here it is:

2;0 juerd@ouranos:~$ perl -MO=Deparse,-p -e'(@dir) = readdir(ID) || pr
+int "X"'
(@dir = (readdir(ID) || print('X')));
-e syntax OK
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The -p adds extra parens. Documentation can be found at B::Deparse.

++ vs lbh qrpbqrq guvf hfvat n ge va Crey :)
Nabgure bar vs lbh qvq fb jvgubhg ernqvat n znahny svefg.
-- vs lbh hfrq OFQ pnrfne ;)
    - Whreq