Better yet:

use Fcntl qw(:flock);

binmode FILE;
eval { flock FILE, LOCK_EX };
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eval will catch the exception caused by using flock where not supported and keep the program from dying. Just checking for $opsys eq "unix" is not sufficient - Perl does have flock on Windows NT f.ex, and there might be pathological cases where a system that identifies itself as unix does not. Catching the exception is a robust way to have it work wherever possible.

Makeshifts last the longest.