Programming Perl, 3rd Edition

Chapter 33. Diagnostic Messages

These messages are classified as follows (listed in increasing order of desperation):

Class	Meaning
(W)	A warning (optional)
(D)	A deprecation (optional)
(S)	A severe warning (mandatory)
(F)	A fatal error (trappable)
(P)	An internal error (panic) that you should never see (trappable)
(X)	A very fatal error (nontrappable)
(A)	An alien error message (not generated by Perl)

The majority of messages from the first three classifications above (W, D, and S) can be controlled using the warnings pragma or the -w and -W switches. If a message can be controlled by the warnings pragma, its warning category is given after the classification letter; for example, (W misc) indicates a miscellaneous warning. The warnings pragma is described in Chapter 31.

Warnings may be captured rather than printed by setting $SIG{__WARN__} to a reference to a routine that will be called on each warning. You can also capture control before a trappable error "dies" by setting $SIG{__DIE__} to a subroutine reference, but if you don't call die within the handler, the exception is still thrown when you return from it. In other words, you're not allowed to "de-fatalize" an exception that way. You must use eval for that.

Default warnings are always enabled unless they are explicitly disabled with the warnings pragma or the -X switch.

In the following messages, %s stands for an interpolated string that is determined only when the message is generated. (Similarly, %d stands for an interpolated number--think ...