I'd suggest separating parsing from evaluation. The first argument to the action subroutines is a hash that you can use for any purpose. In this case, I'd store parts of the specification in there, then return that hash (which makes the convenient default ::= action => ::first work for the Dice_Expression rule). Also, I split modifier_r_comp from modifier_r action, but you could equally merge them and count the number of arguments.

use Marpa::R2;
use Data::Dump;

my $dsl = <<'END_OF_DSL';

:default ::= action => ::first
lexeme default = latm => 1

Dice_Expression ::= Dice_Expression1
                  | Dice_Expression1 add_modifier

Dice_Expression1 ::= Simple_Dice
                   | Simple_Dice x_modifier
                   | Simple_Dice r_modifier

Simple_Dice  ::= Rolls 'd' Sides                         action => sim
+ple_roll
add_modifier ::= '+' Die_Modifier_Val                    action => mod
+ifier_add
               | '-' Die_Modifier_Val                    action => mod
+ifier_add
x_modifier   ::= 'x' Die_Modifier_Val                    action => mod
+ifier_x
r_modifier   ::= 'r' Die_Modifier_Val                    action => mod
+ifier_r
               | 'r' Die_Modifier_Comp Die_Modifier_Val  action => mod
+ifier_r_comp


Die_Modifier_Val ~ digits
Die_Modifier_Comp ~ 'gt' | 'lt'

Rolls ~ digits
Sides ~ digits

digits ~ [\d]+

:discard ~ whitespace
whitespace ~ [\s]+
END_OF_DSL

my $grammar = Marpa::R2::Scanless::G->new( { source => \$dsl } );
my $input = $ARGV[0] // '6d4x1';
my $parsed = $grammar->parse( \$input, 'My_Actions' );
print "\n\nParsed result: ";dd $parsed;

# print "\n\nFinal result: ";dd evaluate_rolls($parsed);

sub evaluate_rolls {
    my $spec = shift;
    # TODO...
}

sub My_Actions::modifier_add {
    my ( $self, $sign, $val ) = @_;
    $$self{add} = 0 + "$sign$val";
    $self;
}

sub My_Actions::modifier_r {
    my ( $self, undef, $reroll ) = @_;
    $$self{r} = $reroll;
    return $self;
}

sub My_Actions::modifier_r_comp {
    my ( $self, undef, $comp, $reroll ) = @_;
    $$self{comp} = $comp;
    $$self{r} = $reroll;
    return $self;
}

sub My_Actions::simple_roll {
    my ( $self, $rolls, undef, $sides ) = @_;
    $$self{rolls} = $rolls;
    $$self{sides} = $sides;
    return $self;
}

sub My_Actions::modifier_x {
    my ( $self, $modifier, $modifier_val ) = @_;
    $$self{x} = $modifier_val;
    return $self;
}
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Update: Added +/- modifiers