Your example script will never give such an error since you never tell Perl that any of your data might be anything other than Latin-1 bytes. So Perl reads and writes the data as Latin-1 bytes and sees no reason to warn.

You can only get this warning when you use Encode::decode explicitly or when you (or a module you use) decode your data from a character set encoding to Unicode implicitly, for example by

    open my $fh, '<:encoding(UTF-8)', $filename
        or die "Couldn't open '$filename': $!";
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Maybe Perl 5.24 warns in more situations than Perl 5.16 did when writing Unicode strings to a file (or somewhere else).

To correct the situation Perl warns about, be explicit in your output encoding and open your output files as

    open my $fh, '>:encoding(UTF-8)', $filename
        or die "Couldn't open '$filename': $!";
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... or encode your strings to bytes before writing them:

use charnames ':full';
use Encode 'encode';

open my $fh, '>', $filename
    or die "Couldn't create '$filename': $!";

my $output = "MOT\N{CAPITAL LETTER O WITH DIAERESIS}RHEAD";
my $output_bytes = encode( 'UTF-8', $output );
print $fh $output_bytes;
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