$ perl5.10.0 -w
sub when { use POSIX "strftime"; strftime("[%Y-%m-%d %H:%M:%S] ", gmtime) }
warn when, "some warning";
__END__
[2007-12-23 20:51:11] some warning at - line 2.
$ perl5.10.0 -w
use feature ":5.10";
sub when { use POSIX "strftime"; strftime("[%Y-%m-%d %H:%M:%S] ", gmtime) }
warn when, "some warning";
__END__
syntax error at - line 3, near "warn when"
Execution of - aborted due to compilation errors.