Perl: fix the // operator

Summary:
BUG: 407327

The "/" character is used for patterns and this causes the "//" operator not to be highlighted correctly in some situations, such as after identifiers.

Reviewers: Framework: Syntax Highlighting, dhaumann, cullmann

Reviewed By: Framework: Syntax Highlighting, dhaumann, cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D21286