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$ perl -le '$_ = join "-", %h = (1,2,3,4,1,2,3,4); print "yes" if /--/
+'
yes

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map { $_ } %h = ( 1,1, 1,1 );    # no undefined values. 
grep { 1 } %h = ( 1,1, 1,1 );    # undefs occur.
sort %h = ( 1,1, 1,1 );          # undefs occur.
reverse %h = ( 1,1, 1,1 );       # undefs occur.