It's two negation operators. In scalar context ("!" imposes a scalar context), grep returns the number of matching elements. Negating this twice converts zero to false, and non-zero to true. False is 0 when used as a number. True is 1 when used as a number.

For example,

                   | Ex. 1 | Ex. 2
-------------------+-------+------
grep returns       | 4     | 0
1st negation op    | false | true
2nd negation op    | true  | false
When used as a num | 1     | 0
When used as a str | '1'   | ''
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