use File::Pid;

# Check to make sure we are not already running

my $pidfile = File::Pid->new({file => "/path/to/my.pid"});

# This next line gets the current pid in $pidfile 
# If nothing was running, we should get back our own pid

my $pid = $pidfile->write;   

# now, die if the pid file cannot be opened, 
# or the pid running is not THIS INSTANCE 
# note: this can be overridden if $FORCE is true.

die ("running process=".$pid.", my pid=$$\n") if (!$pid or ($pid != $$ and !$FORCE));

# ... do a bunch of stuff for a long time

$pidfile->remove or warn "Couldn't unlink pid file\n";