Sorry. I misunderstood your requirement (though it could have been stated a little more clearly:^).
This one (I think does what you want, edge case as well and is relatively simple.
$s = "xx|xx|xxxx|xx|xxx|xx|xx|xx";
$s =~ s[(?<!x)(x)(x)(?!x)][$1 $2]g;
x x|x x|xxxx|x x|xxx|x x|x x|x x
Update Thinking about it, stick with tachyon's as this will fall over if your data contains any occurance of xx without |'s or x's on either side.
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