Interesting that there's also a discrepancy with 1e-298 on perl's (such as yours) whose nvtype is 'double'.
When I check on my 'double' build of perl 5.18.0, I also find a discrepancy.
The script I originally posted is not properly portable to 'double' builds of perl. This rendition of the script should work as intended for both nvtypes:

#!perl -l
use warnings;
use strict;
use Config;

use Inline C => Config =>
  USING => 'ParseRegExp',
  BUILD_NOISY => 1;

use Inline C => <<'EOC';

SV * get_problem_val(long nvsize) {
  if(nvsize == 8) return newSVnv(1e-298);
  return newSVnv(1e-298L);
}

EOC

my $s = $Config{nvsize};
print "\n\$Config{nvtype}: $Config{nvtype}";
print "Different values" if 1e-298 != get_problem_val($s);

if($Config{nvsize} != 8) {
print scalar reverse unpack "b64", pack "D", 1e-298;
print scalar reverse unpack "b64", pack "D", get_problem_val($s);
}
else {
print scalar reverse unpack "b53", pack "F", 1e-298;
print scalar reverse unpack "b53", pack "F", get_problem_val($s);
}
[download]

Thanks, Ken.

Cheers,
Rob