pragma for lexically disabling autovivification

 autovivication is a Perl pragma that enables developers to control whether
 variables can be created automatically on their first use, rather than
 requiring them to be defined beforehand. While occasionally useful, this
 behaviour can result in subtle bugs that are difficult to debug.
 .
 This pragma lets you disable autovivification for some constructs and can
 optionally also emit a warning or error when it would have happened.