Some solutions, from best to worst:

1) Why aren't you using flock? Create the lockfile if it doesn't exist, then flock it. The lock will be cleared if the process exits by any means. Don't worry about deleting the lockfile (but you can if you want).

2) Another way is to store the locker's pid in the lockfile. To check if the lock is active check for both the presence of the lockfile and the presense of the process mentioned therein. If the process isn't running, the lock must be stale.

3) Use END. This will work if the script dies, but it won't work if the script is killed.

{
   my $lock = ...;
   my $on_exit = sub { releaseLock($lock); };
   eval 'END { $on_exit->() }';
}
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You shouldn't modify the END line at all, lest you risk introducing bugs and security problems. Make your changes to on_exit instead.