flock

flock FILEHANDLE, OPERATION

This function calls flock(2) on FILEHANDLE. See the manual page for flock(2) for the definition of OPERATION. Invoking flock will produce a fatal error if used on a machine that doesn't implement flock(2) or emulate it through some other locking mechanism. Here's a mailbox appender for some BSD-based systems:

$LOCK_SH = 1; $LOCK_EX = 2; $LOCK_NB = 4; $LOCK_UN = 8; sub lock {
 flock MBOX, $LOCK_EX; # and, in case someone appended # while we were waiting... seek MBOX, 0, 2;
}
sub unlock {
 flock MBOX, $LOCK_UN;
}
open MBOX, ">>/usr/spool/mail/$ENV{'USER'}" or die "Can't open mailbox: $!"; lock();
print MBOX $msg, "\n\n"; unlock();

Note that flock is unlikely to work on a file being accessed through a network file system.