If you're executing a different executable, i.e. you're calling execve or another of the exec familiy of function, perhaps indirectly through a function like system or popen, and the child process should run without privileges from the start, then the simplest way is to get a shell involved, and call su. Here's an overview of how the code might look like in Perl, showing the quoting needed for :

$shell_command = quotemeta($path_to_executable) . " --option";
$shell_command =~ s/'/'\\''/; # protect single quotes for the use as argument to su
$su_command = sprintf("su -c '%s' %s", $shell_command, quotemeta($user_name));
open(PIPE, "$su_command |") or die;

If the child process needs to start as root but drop privileges later, see the code in this answer, which illustrates how to downgrade privileges in a process.

If you're executing a different executable, i.e. you're calling execve or another of the exec familiy of function, perhaps indirectly through a function like system or popen, and the child process should run without privileges from the start, then the simplest way is to get a shell involved, and call su. Here's an overview of how the code might look like in Perl, showing the quoting needed for :

$shell_command = quotemeta($path_to_executable) . " --option";
$shell_command =~ s/'/'\\''/; # protect single quotes for the use as argument to su
$su_command = sprintf("su -c '%s' %s", $shell_command, quotemeta($user_name));
open(PIPE, "$su_command |") or die;

If the child process needs to start as root but drop privileges later, see the code in this answer, which illustrates how to downgrade privileges in a process.