Guessing is sometimes fine, but the OP gave an entire and complete test script which you can examine and even run. If I make a small modification to the test script given we can test the theory:

# test.pl
use warnings;
$template = 'd<';
$nv = 2.4;
$p = pack $template, $nv;
$s = "'$p'";
print ">$s<\n";

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

>'333333@'<
3333333333330340
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running a 64 bit build of 5.32. Note that there is an ETX character between the final 3 and the @ character, which, when you think about it, is the string that the unpack is showing us so for the success case there is a trailing @ and it isn't swallowing the trailing '.