my $LOCK_EX = 2;       # exclusive lock on file
my $LOCK_UN = 8;       # unlock file

my $lock_file = $ENV{ XYZ_DIR } . "/$0.lock";

open my $lh, ">$lock_file" || die "Can't open lock file $lock_file: $!";
flock $lh, $LOCK_EX; # will wait for exclusive lock
                     # on lock file

# do the exclusive actions

flock $lh, $LOCK_UN;
close $lh;