Environment variable values may contain every character except ASCII NUL ("\0"), because they are defined as ASCII NUL terminated C strings. I wonder how you define $SEPARATOR when the code on the C/C++ side returns a single string also terminated with ASCII NUL.

The only clean way to stuff the entire environment into a single C/C++ string is to use some kind of encoding, not a simple join. You need either a length information before each part of the environment, so that you do not need the trailing ASCII NUL after each key-value-pair; or you replace the trailing ASCII NUL after the pairs with an escape sequence (e.g. the characters "\" and "0"), and replace every occurance of the escape character in the key or value with a second escape sequence (e.g. by using the escape character twice).

Of course, the Perl code has to read the length information and use substr accordingly resp. undo those escaping. Perl uses "counted" strings that can contain ASCII NUL, so unescaping simply means replacing \\ with \ and \0 with ASCII NUL at the same time, followed by splitting at ASCII NUL.

Alexander