my $lockfile = '/var/lock/mylockfile';
open L, $lockfile;
close L;
die "Can't generate lockfile $!" unless -e $lockfile;

my $pid = fork();

if ( $pid == 0 ) {
    # child
    my $error = update_somefile();
    die $error if $error;
    unlink $lockfile or die $!;
    # more stuff

} elsif ( $pid ) {
    # parent
    my $timeout = 0;
    while ( -e lockfile ) {
        sleep 1;
        $timeout++;
        die "of old age waiting for child to finish update" if $timeout > $whatever_time;
    }
    # lockfile gone, child has completed update so...

} else {
    die "Fork failed!\n";
}