open(STDIN,"</dev/tty")
##</code><code>##

open(STDIN,"/dev/null");
#do some ops

if((my $pid = fork()) == 0) {
    # child here
    open(STDIN,"</dev/tty") or die $!;
    # do some ops...
    exit;
} else {
    # parent here - STDIN still is /dev/null
    # more ops...
}
# any code hereafter will be executed by parent and,
# if the exit in the child block wasn't present,
# by the child also