This is a guess but I've added an explicit HTTP::Headers object, set the content-type and included your XML content directly in the HTTP::Request constructor. I used something I previously worked on as a reference. I hope this helps.

require HTTP::Headers;
my $header = HTTP::Headers->new();
$header->header( 'Content-Type' => 'text/xml' );

require HTTP::Request;
my $postRequestObject = HTTP::Request->new(
  POST => $POST_URL,
  $header,
  [ CLRCMRC_XML => $finalXML ] );
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