Support fat comma in Perl lexer

[pyrmont]

The => operator (sometimes called the 'fat comma') is an operator in Perl that causes the identifier to the immediate left of the fat comma to be treated as if it were a string. See additional explanation in the answers to this StackOverflow question.

This fixes #1164.

[pyrmont]

@miparnisari How does this look to you?

[pyrmont]

@jneen This PR now correctly lexes the following:

# fat commas                                  
use constant EMPTY_SPACE => ' ';
my %h = ( FOO => 23 );
$text = format_text({FOO => 23, BAR => 30});
foo(
  sub # comment
      # second comment
    => 1
)

It does this using a regular expression similar to the one you suggested:

rule(/(?=[a-z_]\w*(\s*#.*)*\s*=>)/i) { push :fat_comma }

This looks good to me but if you can think of any issues, let me know.

[pyrmont]

@jneen I realised that the previous rule would match comments that contained a fat arrow. This variation fixes that:

rule(/(?=[a-z_]\w*(\s*#.*\n)*\s*=>)/i) { push :fat_comma }