#!/usr/bin/env perl

use strict;
use warnings;

$^I = '.bak';

my $head_len = 5; 
my $foot_len = 3; 

while (<>) {
    chomp;
    print substr($_, $head_len, length($_) - $head_len - $foot_len), "\n";
}

##</code><code>##

$ ls -l pm_1076973.*
-rwxr-xr-x  1 ken  staff  210  5 Mar 08:50 pm_1076973.pl
-rw-r--r--  1 ken  staff   87  5 Mar 08:46 pm_1076973.txt

##</code><code>##

$ cat pm_1076973.txt
12345... content ...123
12345... more content ...123
12345... even more content ...123

##</code><code>##

$ pm_1076973.pl pm_1076973.txt

##</code><code>##

$ ls -l pm_1076973.*
-rwxr-xr-x  1 ken  staff  202  5 Mar 08:56 pm_1076973.pl
-rw-r--r--  1 ken  staff   63  5 Mar 08:56 pm_1076973.txt
-rw-r--r--  1 ken  staff   87  5 Mar 08:46 pm_1076973.txt.bak

##</code><code>##

$ cat pm_1076973.txt
... content ...
... more content ...
... even more content ...

##</code><code>##

$ cat pm_1076973.txt.bak
12345... content ...123
12345... more content ...123
12345... even more content ...123