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$ perl -Mstrict -Mwarnings -e ' use List::Util qw{first}; ... 1-19 3 20-29 2 30-39 0
$ perl -Mstrict -Mwarnings -e ' my @input = qw{12 14 15 20 21}; ... 1-19 3 20-39 2 40-59 0