Programming Perl, 4th Edition

warn

warn LIST
warn

This function produces an error message, printing LIST to STDERR just like die, but it doesn’t try to exit or throw an exception. For example:

warn "Debug enabled" if $debug;

If LIST is empty and $@ already contains a value (typically from a previous eval), the string “\t...caught” is appended following $@ on STDERR. (This is similar to the way die propagates errors, except that warn doesn’t propagate [reraise] the exception.) If the message string supplied is empty, the message “Warning: Something's wrong” is used.

As with die, if the strings supplied don’t end in a newline, file and line number information is automatically appended. The warn function is unrelated to Perl’s –w command-line option, but can be used in conjunction with it, such as when you wish to emulate built-ins:

warn "Something wicked\n" if $^W;

No message is printed if there is a $SIG{_ _WARN_ _} handler installed. It is the handler’s responsibility to deal with the message as it sees fit. One thing you might want to do is promote a mere warning into an exception:

local $SIG{_ _WARN_ _} = sub {
    my $msg = shift;
    die $msg if $msg =~ /isn't numeric/;
};

Most handlers must therefore make arrangements to display the warnings that they are not prepared to deal with, by calling warn again in the handler. This is perfectly safe; it won’t produce an endless loop because _ _WARN_ _ hooks are not called from ...