unless (-e $file) {
  open FILE, "> $file";
  # ...
}

##</code><code>##

use POSIX qw( O_EXCL O_CREAT );

$perm = 0644;  # or whatever
sysopen FILE, $file, O_EXCL | O_CREAT, $perm or die "can't create $file: $!";