I want to open a file, read it's contents line by line, perform a regex on each line, make a substitution if necessary, write it to the file, then close the file.

consider this bit of code:

use Tie::File;
use strict;

my @ry=();

# I'm not sure where $foo came from in your OP...

tie @ry,"Tie::File","$foo" or die "$foo -- $!";

$_ =~s/hello/goodbye/gi foreach @ry;
untie @ry;
[download]