Emits a warning, usually by printing it to STDERR. warn interprets its operand LIST in the same way as die, but is slightly different in what it defaults to when LIST is empty or makes an empty string. If it is empty and $@ already contains an exception value then that value is used after appending "\t...caught". If it is empty and $@ is also empty then the string "Warning: Something's wrong" is used.
By default, the exception derived from the operand LIST is stringified and printed to STDERR. This behaviour can be altered by installing a $SIG{__WARN__} handler. If there is such a handler then no message is automatically printed; it is the handler's responsibility to deal with the exception 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 upSee 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.