As a pragma:

use diagnostics [-verbose]

enables the use of diagnostics in your program (and enables Perl’s -w flag). Compilation is then subject to the enhanced diagnostics, which are issued to STDERR. Set the diagnostics::PRETTY variable in a BEGIN block to provide nicer escape sequences for pagers, so your output looks better. The -verbose option prints the perldiag manpage introduction, then any diagnostic messages. Functions are: