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 - $!";
}