Patterns such as s!...!...! (in place of the traditional s/.../.../) are not highlighted properly.
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It's a feature, not a bug ;)
Seriously, Perls syntax is so extensive that not everything has been implemented in order to keep highlighting moderately responsive. In this particular case, the syntax highlighting engine would ideally support for some form of backreferences to detect the character that started the expression. Since we don't have these, the patterns would have to be duplicated for each allowed delimiter. If you feel up to it, you could try to duplicate the regex patterns for your favorite alternative delimiter.