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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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prints warnings about dubious constructs, such as variable names that are mentioned only once and scalar variables that are used before being set, redefined subroutines, references to undefined filehandles or filehandles opened read-only that you are attempting to write on, values used as a number that don't look like numbers, using an array as though it were a scalar, if your subroutines recurse more than 100 deep, and innumerable other things.

This switch really just enables the internal $^W variable. You can disable or promote into fatal errors specific warnings using __WARN__ hooks, as described in 10 and . See also 25 and "Perl traps for the unwary" (perltrap) in Perl Tutorials. A new, fine-grained warning facility is also available if you want to manipulate entire classes of warnings; see "Perl pragma to control optional warnings" (warnings) in the Perl Library Reference Manual (Volume 1) or 24.

ISBN 9781906966027Perl Language Reference ManualSee the print edition