You decoded the characters on input, but you didn't encode them on output.

open IN, '<:encoding(UTF-8)', "WE_EX.txt"
open OUT, '>:encoding(UTF-8)', "WE_EX.out"
[download]

You don't have a infinite loop. The program is waiting for input from the keyboard because you are reading from ARGV instead of IN.

while (<IN>) {
[download]

You are needlessly using global variables for your file handles.

use strict;
use warnings;

open(my $IN, '<:encoding(UTF-8)', 'WE_EX.txt')
    or die "Can't open file WE_EX.txt for reading: $!";

open(my $OUT, '>:encoding(UTF-8)', 'WE_EX.out')
    or die "Can't open file WE_EX.out for writing: $!";

while (<$IN>) {
    # ... Manipulate $_ ...
    print $OUT $_;
}

close $IN;
close $OUT;
[download]