[tag-expressions] Fix Perl handling of backslash escaping in tags

[ehuelsmann]

Summary

The escaping backslash was added to the tag and if the escaped character
was a space, it was skipped even though being escaped.

Motivation and Context

Review of the Perl implementation triggered by #1778.

How Has This Been Tested?

More unit tests have been added.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• The change has been ported to Java.
• The change has been ported to Ruby.
• The change has been ported to JavaScript.
• The change has been ported to Go.
• The change has been ported to .NET.
• I've added tests for my code.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have updated the CHANGELOG accordingly.

Adding to CHANGELOG not necessary: the Perl implementation hasn't been released yet.