use Fcntl qw(:DEFAULT :flock);

my $path = "/path/to/data/";
my $pattern = "[0-9]" x 7;

# Every program which creates a datfile here must honor dat.lock
# take the lock before we read names
my $pathlock = "${path}dat.lock";
open LOCK, "> $pathlock"  or die $!;
flock LOCK, LOCK_EX   or die $!;

my @lastfile = sort glob("${path}${pattern}.dat");
my ($nextnum) = $lastfile[-1] =~ /(\d{7})\.dat$/;
$nextnum += 1;
my $nextname = sprintf "%07d.dat", $nextnum; # answers question 2

open NEWDAT, "> ${path}${nextname}" or die $!;
# ...
close NEWDAT or die $!;
close LOCK or die $!;