Now, yesterday, I ran into an issue that involved quite low-level transformations and as it turned out (thanks tofjw!), my string value was simply ignored and a *true* numeric value worked.

Really? If that's true, then there's a bug in Perl. Perhaps you can show a code fragment that shows this behaviour?

Now: what is the best way to transform a string to a true numeric value, in terms of speed/internal efficiency?

You tell us. Run a benchmark and tell us. Note also that they aren't equivalent if $string contains, say "1.23". Of course, I would expect that the fastest solution is to do nothing, and to let perl decide if and when a conversion is needed. And way faster still would be the get rid of Perl, ditch the CGI interface, and write embedded C.