Once again IRC help, so for the sake of completeness I’m answering myself: the reason for the awkward difference is compile time optimisation. Where, in the first case, that last concatenation & stringification still happened during initial unset locale.

Note how the last two prints work as expected if one replaces the constant with a subroutine call.

~$ perl -E 'sub x { 3.14 }; use POSIX qw(setlocale LC_NUMERIC); say x;
+ setlocale(LC_NUMERIC, ""); say x; setlocale(LC_NUMERIC, "C"); say x;
+ setlocale(LC_NUMERIC, ""); say x; say x . ""; ' 
3.14
3,14
3.14
3,14
3,14
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