You would usually say

{
   local $#; 
   ... do some stuff
}
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to have e.g. $# restored at the end of a block.

To force something to stringize as a number, first convert it with the 0+ operator and then use it in a string context:$x = "0.";  $x = 0+ $x;  print $x; Just kidding, there is no 0+ operator; but simply adding 0 as above will produce something with a numeric value and no string value; then using it in string context will cause the numeric value to be stringized, invoking $# (unless the newish preserve-integers-where possible logic kicks in).

By the way, $# is deprecated; use printf/sprintf directly instead; that will save you having to do the conversion: $x = "0."; printf "%.15e", $x and will avoid the aforementioned problem with perl bypassing floating point.