lc() does not modify the string you passed in, what you should do is to assign what returned from lc() back to $big_string.

If you add "use warnings" to the begining of your script. You will see:

Useless use of lc in void context at foo line bar.
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Update:

Just to add one piece of sample code:

use strict;
use warnings;

my $a = "AbCdEfG";
$a = lc($a);
print $a;
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Update 2:

How about "use strict", and declare your variables.

So you expect your file to be one-liner, don't you? As you only read one line from it.