What were my request headers?

echo_headers.pl

# plackup echo_headers.pl

use strict;
use warnings;

use Plack::Request;
use JSON;

my $app = sub {
    my $req = Plack::Request->new(shift);
    my $res = $req->new_response(200);
    $res->body( JSON->new->pretty->encode( {
        map  { $_ => $req->env->{$_} }
        grep { $_ =~ /^HTTP/ } keys %{ $req->env }
    } ) );
    $res->content_type('application/json');
    $res->finalize;
};