#!/usr/bin/perl -w

use Fcntl qw(O_WRONLY O_CREAT O_EXCL);
my $lockfile = '/tmp/lock';
if (sysopen(FH, $lockfile, O_WRONLY | O_CREAT | O_EXCL)) {
    `your_update_script`;
    close FH;
    unlink $lockfile or die "Can't unlink lockfile $lockfile $!";
    print "Updated";
}
else {
    die "Update already in progress. Aborting!";
}