See the exec family of functions, e.g. execl:

  int execl(const char *pathname, const char *arg, ...);

Normally you would call it like execl("program", "program", "argument", NULL), duplicating the program's name/path, but it's possible to set the 2nd argument, which corresponds to argv[0] (in the case of execl), to something different. This behaviour just isn't always exposed in the shell and other languages.

The command line version of exec offers "-a name", and the various library/syscall forms allow you to specify argv[0] separately from the path of the binary to exec.

You can set it yourself within a running program as well, like:

  perl -e '$0="testme";while(1){sleep 1}' &
  ps -ef | grep testme

argv[0] is independent of the executable path. In fact there’s a common “trick” on Unix systems where you offer one binary and it figures out what it is supposed to be by peeking at argv[0]. Also, you’re allowed to freely modify argv, so if anyone reads it by scanning your stack it’ll get the new value.

Man 3 exec