warn

warn

warn LIST

Prints the value of LIST to STDERR. If the last element of LIST does not end in a newline, it appends the same file/line number text as die does-

If the output is empty and $@ already contains a value (typically from a previous eval) that value is used after appending "\t...caught" to $@ . This is useful for staying almost, but not entirely similar to die-

If $@ is empty then the string "Warning: Something's wrong" is used.

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 (like, for instance, converting it into a die)- Most handlers must therefore arrange to actually display the warnings that they are not prepared to deal with, by calling warn again in the handler- Note that this is quite safe and will not produce an endless loop, since __WARN__ hooks are not called from inside one.

You will find this behavior is slightly different from that of $SIG{__DIE__} handlers (which don't suppress the error text, but can instead call die again to change it)-

Using a __WARN__ handler provides a powerful way to silence all warnings (even the so-called mandatory ones). An example:

# wipe out *all* compile-time warnings
BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20;          # no warning about duplicate my $foo,
                       # but hey, you asked for it!
# no compile-time or run-time warnings before here
$DOWARN = 1;

# run-time warnings enabled after here
warn "\$foo is alive and $foo!";     # does show up

See perlvar for details on setting %SIG entries and for more examples. See the Carp module for other kinds of warnings using its carp() and cluck() functions.