From the perldelta for Perl v5.22.0:

A literal { should now be escaped in a pattern

If you want a literal left curly bracket (also called a left brace) in a regular expression pattern, you should now escape it by either preceding it with a backslash (\{) or enclosing it within square brackets [{], or by using \Q; otherwise a deprecation warning will be raised. This was first announced as forthcoming in the v5.16 release; it will allow future extensions to the language to happen.

But ... what is grep /$_/, @wf supposed to be doing?

In reply to Re: "Unescaped left brace in regex is deprecated" by Anonymous Monk
in thread "Unescaped left brace in regex is deprecated" by stevieb

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