use utf8;
use open ':std', ':utf8';
use feature 'unicode_strings';

...

for my $file ( @files ) {

    open my $FH, '<', $file or die "Cannot open '$file' because: $!";
    my $size = read $FH, my $data, -s $FH or die "Cannot read from '$file' because: $!";
    $size == length( $data ) or die "Error reading from '$file'\n";
    close $FH;

...

##</code><code>##

    $size == $data =~ y///c or die "Error reading from '$file'\n";