I am authenticating through Rackspace API with the following code (using LWP):

sub authenticate {

    # Create a request
    my $req =
      HTTP::Request->new(
        POST => 'https://identity.api.rackspacecloud.com/v2.0/tokens' 
+);
    $req->content_type('application/json');
    #my $json = '{"auth":{"RAX-KSKEY:apiKeyCredentials":{"username":"f
+oo", "apiKey":"00000000000000000000000000000000"}}}';
    my $json = quotemeta( {"auth":{"RAX-KSKEY:apiKeyCredentials":{"use
+rname":$USERNAME, "apiKey":$API_KEY}}} );
    $req->content( $json );

    # Pass request to the user agent and get a response back
    my $res = $ua->request($req);

    # Check the outcome of the response
    if ( $res->is_success ) {
        # Do stuff ...
    } else {
        print $res->status_line, "\n";
    }

}
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The problem is that I am not able to interpolate the variables in JSON string ($USERNAME and $API_KEY) - I am getting 400 BadRequest response. I have tried quotemeta (in the code snippet above), \Q and \E operators, escaping JSON double quotes with \.

The commented out $json line (with hard-coded credentials) works just fine.