$i appears only once, the behaviour is defined 
$ perl -MO=Deparse,-p -le"print - --$i"
BEGIN { $/ = "\n"; $\ = "\n"; }
print((-(--$i)));
-e syntax OK

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You were thinking of 
print(-( ++$i - --$i ))

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In reply to Re^4: Boolean counter? by Anonymous Monk
in thread Boolean counter? by DreamT

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