in reply to Regex to only show 2nd character of the same character in a string

my @chars = grep 0==(++$seen{$_})%2, /./sg;

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For example, 

$ perl -E'say for grep 0==(++$seen{$_})%2, "abccabbcabc" =~ /./sg;'
c
a
b
b
c

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In Section Seekers of Perl Wisdom