var leg = {
aa: "a label",
ab: "another label",
....
};
var map = {
aa : ["R1"],
ab : ["AA1"],
ac : ["AA1", "AA2"],
....
};
var monorows = [
["R6", "c", "1", "CB16", "CB15" ... codes that are listed in map],
...
];

##</code><code>##

use JSON;
use Data::Dump "dd";
my $leg_str =<<'END';
{
"aa": "label1",
"ab": "label2",
...
"er": "last label"
}
END
my $leg = decode_json($leg_str);

##</code><code>##

use WWW::Mechanize::PhantomJS;
my $js = WWW::Mechanize::PhantomJS->new();
    $js->get_local("my_js_file.js");
#print $js->content; I get the content ok here.
my ($val, $type) = $js->eval_in_page('function(){JSON.stringify(leg);}');