Looking at the code (grammar.pm) generated by

use Parse::RecDescent;

my $grammar = <<'__EOI__';
parse: <error:FOO!>
__EOI__

Parse::RecDescent->Precompile($grammar, "grammar")
or die("Bad grammar\n");
[download]

reveals (rearranged to avoid forced line breaks)

$_tok = do {
   if (1) {
      do {
         push @{$thisparser->{errors}}, [qq{FOO!},$thisline];
      } unless  $_noactions;
      undef
   } else {
      0
   }
};
[download]

Let's test it:

use Parse::RecDescent;

my $grammar = <<'__EOI__';
parse: {
   # Create error message.
   my $msg = 'FOO!';

   # Create error.
   push(@{$thisparser->{errors}}, [ $msg, $thisline ])
      unless $_noactions;

   # Fail the action.
   undef;
}
__EOI__

my $parser = Parse::RecDescent->new($grammar)
or die("Bad grammar\n");

$parser->parse('');
[download]

Output:

       ERROR (line 1): FOO!
[download]

bingo!