use Fcntl qw(:flock);
#...
# Child does
{
    open my $foo, '+<', $file or die $!;
    flock $foo, LOCK_EX | LOCK_NB or die $!;
    # Reading and Writing
    close $foo or die $!;
}
# child goes on...

# Parent does
sleep 1 while 1 < -M $file;
{
    open my $foo, '<', $file or die $!;
    flock $foo, LOCK_EX or die $!;
    sleep 1;
    unless (kill 0, $child_pid) {
        # close file, restart child, and redo
    }
    # do stuff
    close $foo or die $!;
}