Q3) Does Data::Roundtrip cover the same ground that JSON::XS did? I can't disambiguate what XS means.

Data::Roundtrip requires the JSON distribution. When used, the JSON module will load JSON::XS if it is available to your interpreter, and the pure perl JSON::PP if not. JSON::PP has been in core since ~5.14, so this functionality is very handy instead of just having your script fall over if JSON::XS isn't installed.

XS simply means that there are components to the software that are compiled, therefore in most cases making much of its functionality much, much faster.

One of my distributions, Bit::Manip is an XS based module, but I also have a pure perl version (Bit::Manip::PP) for those who can't or don't want to compile software. Here's a benchmark script between the two, and the results. The C/XS version is ~394% faster than the pure perl version:

use warnings;
use strict;
 
# performs a benchmark between this XS 
# version and the PP version
 
use Benchmark qw(timethese cmpthese);
use Bit::Manip;
use Bit::Manip::PP;
 
my $do = $ARGV[0];
 
timethese(1000000, {
        'c' => 'c',
        'p' => 'p',
    }
);
 
cmpthese(1000000, {
        'c' => 'c',
        'p' => 'p',
    }
);
 
sub c {
    Bit::Manip::bit_set(65535, 0, 8, 0xFF);
}
sub p {
    Bit::Manip::PP::bit_set(65535, 0, 8, 0xFF);
}
 
__END__
 
Benchmark: timing 1000000 iterations of c, p...
         c:  3 wallclock secs ( 3.35 usr +  0.00 sys =  3.35 CPU) @ 29
+8507.46/s (n=1000000)
         p: 17 wallclock secs (16.58 usr +  0.00 sys = 16.58 CPU) @ 60
+313.63/s (n=1000000)

      Rate    p    c
p  60606/s   -- -80%
c 299401/s 394%   --
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