bug#30549: 25.2; perl-mode: font lock improvement

[Eli Zaretskii <eliz@gnu.org>]

> From: Evgeni Kolev <evgeni.d.kolev@gmail.com>
> Date: Tue, 20 Feb 2018 18:19:55 +0200
> 
> I'm reporting an issue with perl-mode's font lock for perl keywords, and
> suggesting a fix.
> 
> To reproduce:
> 
> - emacs -Q
> - clear the scratch buffer
> - M-x perl-mode
> - enter the following text:
> 
>     use Fake;
>     my $use = 123;
>     my $var = 123;
> 
> - observe the issue: `$use` is incorrectly highlighted - it should be
> highlighted with the same color as `$var`, but it's highlighted with the
> same color as `use` (on the first line).
> 
> The fix I suggest is borrowed from cperl-mode - extend the font lock regex
> to *not* match keywords if they start with a perl sigil.

Thanks, pushed to the master branch.

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