A mixture of, mostly minor, stylistic issues:

use strict;
use warnings;
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That mantra should come before you do anything else! Always! Regardless of projet size! No exceptions! Comprende?

(condition) or print "fail string";
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is cute, but

print "fail string" if condition;
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is clearer. In cases where the fail condition is non-zero it is even better to make it explicit:

print "fail string" if 0 != (condition);
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It is tempting to comment things that you have just learned, but that can lead to over commenting and make it harder to grok the flow of the code.

$var1 . 'string1' . "string2" is better written as "${var1}string1string2". Note the ${var1} usage to clarify where the variable name ends.

The block for while (<FILE2CHANGE>) { is not indented.