O => 1; 1 => 0 is the truth table for exclusive disjunction; ie. XOR:

print @a;;
1 1 0 0 1 1 0 0 0 0 0 1 1 1 0 0 

$_ ^= 1 for @a;;

print @a;;
0 0 1 1 0 0 1 1 1 1 1 0 0 0 1 1
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