Perhaps a full example will help:

use strict;
use warnings;
use Marpa::R2;

my $dsl = <<'END_OF_DSL';
lexeme default = latm => 1
 
Dice_Expression ::= Simple_Dice action => ::first
Simple_Dice ::= Rolls ('d') Sides action => do_simple_roll

Rolls ~ digits
Sides ~ digits
digits ~ [\d]+

:discard ~ whiteSpace
whiteSpace ~ [\s]+
END_OF_DSL
 
 
package Actions;

sub do_simple_roll {
    my (undef, $rolls, $sides) = @_;
    my @res = map {1 + int(rand($sides))} 1 .. $rolls;
    
    return \@res;
}

package main;

my $grammar = Marpa::R2::Scanless::G->new({ source => \$dsl});
my $input = '2d6';
my $rolls = $grammar->parse(\$input, 'Actions');
    
print "Result: @$$rolls\n";
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Note the :default ::= action => ::first. Actually we can omit the default line altogether and then change the Dice_Expression line to:

Dice_Expression ::= Simple_Dice action => ::first
[download]

with the same result. The action determines what is passed up to the next level. If there is no action and no explicit default action undef is passed back up (although that isn't what the documentation seems to say). The default action is used where there is no explicit action. Perhaps the key thing to understand is that the called sub gets a parameter in its argument list for each RHS primary (see RHS alternatives) unless the primary is hidden. RHS values are either constant strings or the LHS for a G1 rule, or a L0 symbol.