If you lock a file for writing, it will automatically wait untill any other lock is released. In other words you can do this in both processes and it'll work:

use Fcntl qw(:flock);  # import LOCK_* constants

open F,">>",$somefile or die $!;
flock(F,LOCK_EX);  # lock exclusively - waits for lock

# write stuff here

close(F); # close() will release the lock.
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