open (FILE, "<file.txt");
my @file=<FILE>;
close FILE;
my $file[0]=$file[0] + 1;
open (FILE, ">file.txt");
print FILE "$file[0]\n";
close FILE;

##</code><code>##

tie my @file, 'Tie::File', 'file.txt';
$file[0]=$file[0] + 1;
untie @file;