Probably would make sense to see try to trap the error at the rename and see if the filesystem is doing something weird and not keeping up:

RENAME:{
rename ("ic-fluc.dat" , "ic-fluc-iter-$iter.dat")  and last RENAME;
print "retrying iter $iter\n";
sleep 1;
redo RENAME;
}
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If the file still isn't showing up, wrap the block around your system call to re-execute. -Greg