Perl highlighting broken after a heredoc with "CSS" as the terminator

Consider the example below. When viewed in VSCode, the first "my $a" line is colored correctly while "my $b" is incorrectly colored.

```perl
# Example showing that CSS heredoc is treated differently
my $foo = <<"END";
Heredoc ending in end.
END

# Colored correctly:
my $a = "a";

my $foo = <<"CSS";
Heredoc ending in end.
CSS

# Colored incorrectly:
my $b = "b";
```