int fd = open("/some/file", O_RDWR);
dup2(fd, 7);
close(fd);

##</code><code>##

int pid = fork();
if (pid == 0) {
    execl("/the/perl/program", "program", "arg1", "etc");
    _exit("URK!");
}
close(7);

##</code><code>##

open FH, '+<=7';
my $line = <FH>;       # or whatever