Remember what everybody else said, but I thought I'd point that you would have received an informative message if you had used strict, warnings and diagnostics.

I created a 'counter_data.dat' file with the single character '1' in my current directory, tuned the code a tiny bit (see below), then ran the script. Here's what I got.

Filehandle RECORD_NUMBER opened only for input at io.pl line 8, <RECORD_NUMBER>
        line 1 (#1)
    (W io) You tried to write on a read-only filehandle.  If you intended
    it to be a read-write filehandle, you needed to open it with "+<" or
    "+>" or "+>>" instead of with "<" or nothing.  If you intended only to
    write the file, use ">" or ">>".  See perlfunc/open.

Here is your sample code with the changes I made.

use warnings;
use strict;
use diagnostics;

open (RECORD_NUMBER, "counter_data.dat") or die;
my $count = <RECORD_NUMBER>; # Lexical declaration to keep 'strict' ha
+ppy.
$count = $count + 1;
print RECORD_NUMBER "$count";
close (RECORD_NUMBER) or die;
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I know it can be annoying to include those use statements in all of your code, but it really is helpful. See Use strict warnings and diagnostics or die for a lot of information on helpful ways to make Perl tell you what it's doing.