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print $ENV{HTTP_CONTENT_RANGE};

use warnings;
use strict;
...
print Dumper \%ENV;
print '</pre>';
print "<br>Content-Range: ",$ENV{HTTP_CONTENT_RANGE};

use warnings;
use strict;
...
$req->header( 'Content-Range' => 'bytes 0-499/500' );
my $resp = $ua->request($req);
print  $resp->{_content};