open(my $HTML, '>', 'OUT-access.html') or die "Cannot open file 'OUT-access.html' for writing: $!";
...
print $HTML "There were ";
my $offset = tell $HTML;
printf $HTML "%7d", 0;
print  $HTML " unique visitors in the logfile.<BR>\n";
...
# Calculate $IPcount;
...
seek   $HTML, $offset, 0;
printf $HTML "%7d", $IPcount;