Specifying the input encoding 

use 5.020;
use strict;
use warnings;
use autodie;
use utf8;

my $product_file = 'product-list.txt';

open my $fh, '<:encoding(utf8)', $product_file;

binmode(STDOUT, ':encoding(utf8)');

while ( <$fh> ) {
    s/GBP/£/;
    s/EUR/€/;
    s/USD/\$/;
    print;
}