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       A scalar value is interpreted as TRUE in the Boolean sense if i
+t is not
       the null string or the number 0 (or its string equivalent, "0")
+.  The
       Boolean context is just a special kind of scalar context where 
+no
       conversion to a string or a number is ever performed.

Truth and Falsehood
 
...
       Negation of a true value by "!" or "not" returns a special fals
+e value.
       When evaluated as a string it is treated as '', but as a number
+, it is
       treated as 0.