Update: Nevermind, that was your intention. Sorry, I guess I didn't read your whole post.

The latter doesn't work. You seem to be expecting lazy evaluation, but map processes the entire list before grep is called. 

$ perl -le'print grep { $seen{$_} == 1 } map { $seen{$_}++; $_ } qw( a
+ b a c );'
bc

[download]

Fix: 

grep { ++$seen{$_} == 1 }

grep { !$seen{$_}++ }

[download]

In reply to Re^2: Fast way - get array elements > 4 chars by ikegami
in thread Fast way - get array elements > 4 chars by imrags

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