How do you call the script? It seems you are feeding it with STDIN, which is not affected by use open IO. The following works for me (both in 5.16.2 and 5.10.1): 
#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

binmode STDOUT, 'utf8';
binmode DATA, 'encoding(utf-8)';
while (<DATA>) {
    chomp;
    s/[A-Za-z]//g;
    say $_, ' ', length;
}

__DATA__
æ
æð
æða
æðaber
æðahnútur
æðakölkun
æðardúnn
æðarfugl
æðarkolla
æðarkóngur
æðarvarp
æði
æðimargur
æðisgenginn
æðiskast
æðislegur
æðrast
æðri
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æðruleysi
æðruorð
æðrutónn
æðstur
æður
æfa

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لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: length() miscounting UTF8 characters? by choroba
in thread length() miscounting UTF8 characters? by Anonymous Monk

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