Programming Perl, 4th Edition

The when Statement and Modifier

Most of the power of a given comes from the implicit smartmatching that various data types imply. By default, when(EXPR) is treated as an implicit smartmatch of $_; that is, $_ ~~ EXPR. (See Chapter 3 for more details on smartmatching.) However, if the EXPR argument to when is one of the 10 exceptional forms listed below, it is evaluated directly for a Boolean result, and no smartmatching occurs:

1. A user-defined subroutine call or a method invocation.

2. A regular expression match in the form of /REGEX/, $foo =~ /REGEX/, or $foo =~ EXPR.

3. A smartmatch that uses an explicit ~~ operator, such as EXPR ~~ EXPR. (You might, for instance, want to use an explicit smartmatch against $_ when you need to reverse the default polymorphism of when’s built-in smartmatching.)

4. A relational operator such as $_ < 10 or $x eq "abc" that returns a Boolean result. This includes the six numeric comparisons (<, >, <=, >=, ==, and !=), and the six string comparisons (lt, gt, le, ge, eq, and ne).

5. The three built-in functions defined, exists, and eof.

6. A negated expression, whether !EXPR or not(EXPR), or a logical exclusive or, EXPR1 xor EXPR2. (The bitwise versions [~ and ^] are not included.) Negated regular expressions also fall in this category, whichever way you write them: !/REGEX/, $foo !~ /REGEX/, or $foo !~ EXPR.

7. A file test operator (apart from –s, –M, –A, and –C, as these return numbers, not Booleans).

8. The .. and ... flip-flop operators. (Note that the ... infix operator is completely ...