$line is still encoded. A character won't match the UTF-8 encoding of that character unless it's an ASCII character.

You had the right idea with -C / use open / binmode. The catch is that you're not reading from STDIN, you're reading from ARGV, and those don't work well if at all with ARGV.

The solution: Don't use ARGV.

my %fixes = (
    "\x{00a9}" => '\\textcopyright',
    "\x{2010}" => '-',
    "\x{fffd}" => '\\,',
    "\x{03b4}" => '$\\delta$',
    "\x{00c5}" => '\\AA{}',
);

my ($re) =
    map qr/$_/,
    join '|',
    map quotemeta,
    keys(%fixes);

@ARGV = '-' if !@ARGV;
for my $ARGV (@ARGV} {
    my $fh;
    if ($ARGV eq '-') {
        open($fh, '<&:encoding(UTF-8), *STDIN)
            or die "Can't dup STDIN: $!\n");
    } else {
        open($fh, '<:encoding(UTF-8), $ARGV)
            or die "Can't open \"$ARGV\": $!\n");
    }

    for (;;) {
        last if eof($fh);
        defined( my $line = <$fh> )
            or die("Can't read from \"$ARGV\": $!\n");

        $line =~ s/($re)/$fixes{$1}/g;
        print $line;
    }
}
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Yeah, it sucks. Especially since ARGV normally does that error checking for you.