Hello Pearl12345, and welcome to the Monastery!

You open two filehandles, TEXT for reading and OUT for writing; but then you use only the first. The warning/diagnostic (not error) message is alerting you to the fact that you open OUT but never use it. Most likely, you intended the second-last line to be:

print OUT join("\n", @finalarray);
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BTW, it’s good practice to use strict; and to declare all variable as lexicals (i.e., using my); also, to use lexical variables for filehandles, and to use the three argument form of open:

use strict;
use warnings;
use diagnostics;
open(my $text, '<', "C:/Users/Customer/Desktop/New folder/Perl/1dfre10
+.TXT");
open(my $out,  '>', "C:/Users/Customer/Desktop/New folder/Perl/1dfre10
+.OUT");
undef($/);
my $all_text = <TEXT>;
$all_text = lc($all_text);
$all_text =~ s/[^a-z\-\']/ /g;
my @wordarray = split(/[\n\s]+/, $all_text);
...
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Hope that helps,