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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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Useless use of %s in void context

(W void) You did something without a side effect in a context that does nothing with the return value, such as a statement that doesn't return a value from a block, or the left side of a scalar comma operator. Very often this points not to stupidity on your part, but a failure of Perl to parse your program the way you thought it would. For example, you'd get this if you mixed up your C precedence with Python precedence and said

$one, $two = 1, 2;

when you meant to say

($one, $two) = (1, 2);

Another common error is to use ordinary parentheses to construct a list reference when you should be using square or curly brackets, for example, if you say

$array = (1,2);

when you should have said

$array = [1,2];

The square brackets explicitly turn a list value into a scalar value, while parentheses do not. So when a parenthesized list is evaluated in a scalar context, the comma is treated like C's comma operator, which throws away the left argument, which is not what you want. See 15 for more on this.

This warning will not be issued for numerical constants equal to 0 or 1 since they are often used in statements like

1 while sub_with_side_effects();

String constants that would normally evaluate to 0 or 1 are warned about.

ISBN 9781906966027Perl Language Reference ManualSee the print edition