~/linux/test/perl/library/Regexp/Grammars/</=/\$ cat main.pl 
#!/usr/bin/env perl

use strict;
use warnings;

use Data::Dumper;

my $grammar = do {
  use Regexp::Grammars;
  qr{
    (?#<logfile:->)
    <delimited_string>

    <token: delimited_string>
      <ldelim=str_delim> .*? <rdelim=\ldelim>

    <token: str_delim>
      ["'`]
  }xms;
};

my $string = "''";
if ($string =~ $grammar) {
  print Dumper \%/;
}

~/linux/test/perl/library/Regexp/Grammars/</=/\$ ./main.pl
 error | Found call to <ldelim=str_delim>, but no <rule: str_delim> or
       | <token: str_delim> was defined in the grammar
       | (Did you misspell the rule name or forget to define the rule?)
       | 
 error | Found call to <______0_88_str_delim>, but no <rule: ______0_88_str_delim> or
       | <token: ______0_88_str_delim> was defined in the grammar
       | (Did you misspell the rule name or forget to define the rule?)
       | 
 error | Found call to <str_delim>, but no <rule: str_delim> or
       | <token: str_delim> was defined in the grammar
       | (Did you misspell the rule name or forget to define the rule?)
       | 
  warn | Possible invalid subrule call:
       |     <ldelim=str_delim>
       | (To silence this warning, use: \<ldelim=str_delim>
       | 
  warn | Possible invalid subrule call:
       |     <rdelim=(?{; *Regexp::Grammars::LOGFILE
       | (To silence this warning, use: \<rdelim=(?{; *Regexp::Grammars::LOGFILE
       | 
  warn | Possible invalid subrule call:
       |     <______0_88_str_delim>
       | (To silence this warning, use: \<______0_88_str_delim>
       | 
  warn | Possible invalid subrule call:
       |     <str_delim>
       | (To silence this warning, use: \<str_delim>
       | 
Eval-group not allowed at runtime, use re 'eval' in regex m/(?{; *Regexp::Grammars::LOGFILE
                        = Regexp::Grammars::_open_log('>>','-'); })((?{; @! = () if !.../ at ./main.pl line 10.