You have confused the $string version and the file version of the data that you are editing. Probably the easiest thing to do is to use @Project like this:

#!/usr/bin/perl

use strict;
use warnings;

my @Project = <DATA>;

chomp (@Project); # chomp is a little more selective than chop! It rem
+oves $/

my $width = 80;
my $heading;

for (@Project) {
    if (/(SECTION\s\d+.*)/) {
        print ' ' x (($width-length($1))/2);       # Center the headin
+g
    }
    
    print "$_\n";
}

__DATA__
======================================================================
+=======

SECTION 35 - BUILDING VIM FROM SOURCE


35.1. How do I build Vim from the sources on a Unix system?

For a Unix system, follow these steps to build Vim from the sources:

- Download the source and run-time filles archive (vim-##.tar.bz2) fro
+m the
  ftp://ftp.vim.org/pub/vim/unix directory.
- Extract the archive using the bzip2  and tar utilities using the com
+mand:

        $ bunzip2 -c <filename> | tar -xf -
[download]

Prints:

======================================================================
+=======

                     SECTION 35 - BUILDING VIM FROM SOURCE


35.1. How do I build Vim from the sources on a Unix system?

For a Unix system, follow these steps to build Vim from the sources:

- Download the source and run-time filles archive (vim-##.tar.bz2) fro
+m the
  ftp://ftp.vim.org/pub/vim/unix directory.
- Extract the archive using the bzip2  and tar utilities using the com
+mand:

        $ bunzip2 -c <filename> | tar -xf -
[download]

Note that the sample code avoids needing an external file by using __DATA__. This is a trick worth cultivating for posting questions here.

---

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