Both programs will have the file open for append. If both programs write to them, there's no garantee you end up with something that isn't garbage...

On sane Unix-like systems, there is a guarantee: you won't get garbage, regardless of the size of your writes. [1] If the log file is opened in append mode and each entry is committed as a single write (system call), Unix file semantics guarantee that the entries will be appended to the file atomically. (For more information see Re^3: Looking for a simple multiprocess-enabled logging module, which quotes the relevant bits of the Single Unix Specification.)

See also "All I want to do is append a small amount of text to the end of a file. Do I still have to use locking?" from perlfaq5.

Cheers,
Tom

[1] Provided that your log file resides on a filesystem that supports Unix semantics. Network-mounted filesystems often fall short.