you should be able to influence the value of the Gconvert macro by supplying a value on the ./Configure command line

Excellent!! Following your suggested possible candidates, I found that the configure arg -Dd_Gconvert='sprintf((b),"%.*g",(n),(x))' seems to be doing the trick with perl-5.33.2.

sh Configure -des -Duse64bitall -Dprefix=/home/sisyphus/perl-5.33.2-d 
+-Uversiononly -Dusedevel -Dd_Gconvert='sprintf((b),"%.*g",(n),(x))' &
+& make
.....
Everything is up to date. Type 'make test' to run test suite.
$ ./perl -I./lib -le 'printf "%.54g\n", 0.3;'
0.299999999999999988897769753748434595763683319091796875
sisyphus@sisyphus5-desktop:~/comp/perl-5.33.2$ ./perl -I./lib -le 'pri
+ntf "%.254g\n", 1.7e+251;'
1700000000000000056638039423376108468948472250210191793353461791741809
+721595401553113821670191901996837100770654163405637701161634938299172
+691569638797131996693245937962742921605027934132599151410492365393458
+53734303029546817799537579060533492478640128
[download]

I haven't run make test yet. It's getting late, so I'll leave that until the morning.

Cheers,
Rob