I think split and join would make a better approach here:

while (<FILE>) {
    print join ",", split( " ", $_, 6 );
}
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The first arg in the split call is a "magic" space -- which means break on one or more whitespace characters (update: for the split to use a "magic" space, it needs to be quoted, not a regex -- thanks and apologies, guys!). The third arg to split says that only 6 pieces should be returned -- that means upon seeing the sixth non-whitespace string, it will ignore all remaining whitespace characters, and the sixth element will contain everything to the end of the string.

Update: you might need to watch out for lines in the log file that contain commas; in those cases, double-quotes will be needed around each field that contains a comma. (And if any field contains a double-quote, that needs to be escaped by doubling it:

# input:
Oct  1 13:23:25 smoothwall-swe3 kernel: Enabling fast FPU save, restor
+e and "foo"... done.

# should produce as output:
Oct,1,13:23:25,smoothwall-swe3,kernel:,"Enabling fast FPU save, restor
+e and ""foo""... done."
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Not hard to do if you split into an array, then loop over the array elements. But you may want to look at a module for that (e.g. Text::xSV). Or you could just delete commas and quotes from the input before doing the split/join. ;)