in reply to How to avoid the duplication in a string

Is this homework? 
$ perl -wle '%h = map { print unless defined($h{$_}); $_ => $h{$_}++ }
+ split(//,q{abcdabcdabcd}); '
a
b
c
d

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No matter how great and destructive your problems may seem now, remember, you've probably only seen the tip of them. [1]

In Section Seekers of Perl Wisdom