use utf8 ! :)

update

Without utf8 flag you are forcing perl to transliterate a 2 byte character to a 1 byte character.

My interpretation is that this 1 byte char is doubled to keep the counters in sync.¹

update

same effect here

use warnings;
use strict;
#use utf8;

$_ = '§|§';

print y/§/$/r, "\n";
[download]

Careful with such experiments, I was able to make my Perl (5.14) coredump

update

¹) ikegami's explanation is better, you are transliterating the first and second byte for · separately to .

  DB<123> unpack 'H*','·'
 => "c2b7"
[download]

Got confused about the tr flags and their defaults.