I use this to process s-expressions with Marpa::R2:

my $DSL = <<'DSL';
:default ::= action => ::first
lexeme default = latm => 1

start ::= sexp

sexp ::= list
       | atom

list ::= LPAREN elements RPAREN action => Sexp::Tiny::Actions::do_list
elements ::= element* action => Sexp::Tiny::Actions::do_elements
element ::= sexp

atom ::= string
       | symbol

string ::= DQUOTE string_chars DQUOTE action => Sexp::Tiny::Actions::d
+o_string
         | DQUOTE DQUOTE            action => Sexp::Tiny::Actions::do_
+empty_string

string_chars ::= string_char+ action => Sexp::Tiny::Actions::do_join
string_char  ::= STRCHAR

DQUOTE  ~ '"'
STRCHAR ~ [^"]


symbol ::= symbol_chars action => Sexp::Tiny::Actions::do_symbol
symbol_chars ::= symbol_char+ action => Sexp::Tiny::Actions::do_join
symbol_char  ::= SYMCHAR
SYMCHAR ~ [^()\s"]

LPAREN ~ '('
RPAREN ~ ')'

:discard ~ WS
WS ~ [\s]+

DSL
[download]

Probably awkward and there are certainly better ways, but it works: (i skip the processing part):

karl@pazuzu:~/src/perl/debug/sexp-tiny$ bin/sexp.pl examples/fossil.da
+t                                                                
{  fields => {
              "anonymous"           => "off",
              "canonical-url"       => "https://goethebier.space/fossi
+l/p-acme",
              "crlf-glob"           => "*.bat *.cmd",
              "crnl-glob"           => "",
              "default-user"        => "karl",
              "hash-policy"         => "sha3",              "http-port
+"           => 8081,
              "ignore-glob"         => ["*.o", "*.swp", "*.tmp", ".dir
+env/", "_build/", "_opam/"],
              "localauth"           => "on",
              "max-upload"          => "20MB",              "project-d
+escription" => "Private ACME tooling repository",
              "project-name"        => "p-acme",
              "ssl-ca-location"     => "/etc/ssl/certs",
              "throttle"            => 0,
              "timeline-utc"        => "on",
            },
  type   => "fossil",
}
[download]

But it only works for strings so far:

karl@pazuzu:~/src/perl/debug/sexp-tiny$ echo '(acme (foo "bar"))' | bin/sexp.pl { fields => { foo => "bar" }, type => "acme" }

But not with numbers:

karl@pazuzu:~/src/perl/debug/sexp-tiny$ echo '(acme (x 1))' | bin/sexp
+.pl 
bin/sexp.pl expected (key value...)
[download]

I can only pass them in as string:

karl@pazuzu:~/src/perl/debug/sexp-tiny$ echo '(acme (x "1"))' | bin/se
+xp.pl 
{ fields => { x => 1 }, type => "acme" }
[download]

How must i modify the grammar to handle numbers (float and int)? I didn't find an example yet.

Update:

I'll try to illustrate how i did it:

#!/usr/bin/env perl

use strict;
use warnings;
use lib 'lib';
use Sexp::Tiny::Grammar ();
use Data::Dump          qw(dd);
use Scalar::Util        qw(blessed);

my $src = q{(acme (pi "3.14")(nose "cuke"))};
dd $src;

my $r = Sexp::Tiny::Grammar::build_reader();
dd ref($r);

$r->read( \$src );

my $ast_ref = $r->value;
dd $ast_ref;

sub is_qstr {
    my ($v) = @_;
    return blessed($v) && $v->isa('Sexp::Tiny::String');
}

sub unwrap_qstr {
    my ( $v, $path ) = @_;
    if ( is_qstr($v) ) {
        warn "[unwrap] $path: ", ref($v), qq{ -> plain perl string "},
          $v->value, qq{"\n};
        return $v->value;
    }
    return $v;
}

sub maybe_number {
    my ( $v, $path ) = @_;
    return $v if ref $v;
    if ( defined($v)
        && $v =~ /\A[+-]?(?:\d+(?:\.\d*)?|\.\d+)(?:[eE][+-]?\d+)?\z/ )
    {
        warn "[numify] $path: \"$v\" -> ", 0 + $v, "\n";
        return 0 + $v;
    }
    return $v;
}

sub normalize {
    my ( $node, $path ) = @_;
    $node = unwrap_qstr( $node, $path );
    if ( ref($node) eq 'ARRAY' ) {
        my @out;
        for my $i ( 0 .. $#$node ) {
            push @out, normalize( $node->[$i], "$path\[$i\]" );
        }
        return \@out;
    }
    return maybe_number( $node, $path );
}

my $norm = normalize( $$ast_ref, '$ast' );
dd $norm;
[download]

Looks like it's doing what it's supposed to do:

karl@pazuzu:~/src/perl/sexp-tiny$ leftovers/ex02.pl 
"(acme (pi \"3.14\")(nose \"cuke\"))"
"Sexp::Tiny::Grammar::Reader"
\[
  "acme",
  ["pi", bless(do{\(my $o = 3.14)}, "Sexp::Tiny::String")],
  ["nose", bless(do{\(my $o = "cuke")}, "Sexp::Tiny::String")],
]
[unwrap] $ast[1][1]: Sexp::Tiny::String -> plain perl string "3.14"
[numify] $ast[1][1]: "3.14" -> 3.14
[unwrap] $ast[2][1]: Sexp::Tiny::String -> plain perl string "cuke"
["acme", ["pi", 3.14], ["nose", "cuke"]]
[download]

my $src = q{(acme (pi 3.14)(nose "cuke"))}; works as well.