Parse::RecDescent produces a string containing Perl code which is evaled in a different package (namespace). You need to use charnames in that package:

use strict;
use warnings;

use Parse::RecDescent ();

#$::RD_TRACE = 1;
$::RD_HINT = 1;

my $parser = new Parse::RecDescent(<<'EOG') || die;
   {
      # Put pragmas, modules and functions
      # used by the parser in these curlies.
      use strict;
      use warnings;
      use charnames ':full';
   }

   ex1: "\N{DIAMOND OPERATOR}"
   ex2: "\x{22c4}"
   ex3: /\p{Letter}/
EOG

my $text = chr(0x22C4);

print("$_: ", $parser->$_($text) ? "match" : "no match", "\n")
   foreach qw( ex1 ex2 ex3 );

__END__
output
======
ex1: match
ex2: match
ex3: no match
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Note: \p{Letter} didn't work for me when using ActivePerl 5.6.1 -- Can't find unicode character property definition via main->Letter or Letter.pl. -- but it worked (as shown above) when using ActivePerl 5.8.0.