If Perl, itself, does not internally know a value is a number, how can it convert a number to a string?

Looking at Mojo::JSON and JSON::Tiny, they both seem to be "pure Perl" implementations, so they would have to use heuristics.

Looking at perlapi:

SvNIOK "Returns a U32 value indicating whether the SV contains a number, integer or double."

So, it seems that, internally, Perl knows if a scalar is a number.