$ cat pm_11133064_pack_test.pl
use warnings;

print "OS: $^O; Perl: $^V;\n",
    `perl -V:usequadmath`, `perl -V:nvtype`, "\n";

my @templates = qw{d d< d> D D< D>};

for $template (@templates) {
    print "TEMPLATE: $template\n";
    $nv = 2.4;
    $p = pack $template, $nv;
    $s = "'$p'";

    system $^X, '-wle', "print unpack('H*', $s);";
}