Here are two console scripts. The first does the wrong thing by saving the count in itself. The second does the right thing by using an external data file.

As always in Perl, there are many other ways to increment a count. Neither of these two examples is the worst or best way to do it, they're merely bad and better.

WRONG

#!perl -w
# self-contained counter script increments and prints value in DATA
# set permissions to allow script to read and write itself - epoptai

use strict;

my$e = 0;
my$d;

while(<DATA>){$d .= $_} # read data
$d++; # increment

open (ME,"+>> $0") or die "$!"; # open self
unless($^O=~/mswin/i){ flock(ME,2) or die "$!"} # lock unless win32
my@me = <ME>; # read self into array

for(@me){
    if($_=~m|_\_DATA_\_|){ $e = 1; next } # find last line
    if($e == 1){ $_ =~ s/\d+/$d/o } # and replace with incremented val
+ue
    }
seek ME, 0, 0; # goto top of self
truncate ME, 0; # and clear it
print ME @me; # write modified contents
close ME or die "$!";

print $d; # print incremented value

__DATA__
0
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RIGHT

#!perl -w
# counter script increments and prints value in $file
# set permissions to allow script to read and write $file - epoptai

use strict;

my$file = 'count.txt';

open (FILE,"+>> $file") or die "$!"; # open file
unless($^O=~/mswin/i){ flock(FILE,2) or die "$!"} # lock unless win32
local $/ = undef;
my$num = <FILE>; # read file into array
$num++;

seek FILE, 0, 0; # goto top of file
truncate FILE, 0; # and clear it
print FILE $num; # write modified contents
close FILE or die "$!";

print $num; # print incremented value
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