For generating, say, unique temporary files, use IO::File::new_tmpfile. In addition there's the POSIX::tmpnam() function which would get you a unique filename (subject to a race condition, however, since there's a period between when it's generated and when it's used).

In a pinch, use something like this:

$num = 12345;
$dir = "/some/dir/";

$num++ while -e "$dir/$num";

open(F, ">$dir/$num") or die ...
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But of course, there's still a race condition. A more secure solution would be to take advantage of sysopen and O_EXCL to iterate through filenames:

use Fcntl;
use FileHandle;

$num++ while !sysopen(F, "$dir/$num", O_EXCL|O_CREAT, 0777) && $! eq "
+File exists";

die "$dir/$num: $!" if $!;
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Once you start adding all of the bells and whistles, though, you're just re-implementing IO::File new_tmpfile.