Don't know if using a flag(better: lock)-file is the best way to do it, but it should work for your purpose. Something along...

BEGIN {
 our $LOCKFILE = "/tmp/frogs.pid";  
 die "Another process is already running (see content of $LOCKFILE for
+ pid)...\n" if -e $LOCKFILE;
 open my $out, '>', $LOCKFILE or die "cannot open $LOCKFILE - $!";
 print $out "$$\n";
 close $out;
 # chmod 0600 ... if you like
}

warn "running... with PID: $$ ...\n"; 
sleep 10;

END {
    unlink $LOCKFILE or die "cannot remove lock-file $LOCKFILE - $!";
}
[download]

It is not race-condition proof, but 10-20 minutes in between runs is not a race condition, I guess.
Update: Ok, when using cronjobs, there is a probability not equal to zero that a cronjob may collide one day... interacting with a manual invocation.
Update2: Used /tmp instead of /var/run since you might not have the privileges (root) to access files there. If you do, /var/run is closer to best practises. You might also need to register SIGnal handlers to remove the lock file.