Better / shorter ways to write this? It takes a list and returns a hash of arrays (key => list of indices where key appears in argument) 
use 5.012; use warnings; use Data::Dumper;
sub occurrences {
  my %h;
  while (my ($i, $k) = each (@_)) {
    if (! $h {$k}) { $h {$k} = [] }
    push $h {$k}, $i
  }
  return %h
}

my %h = occurences (2, 1, 3, 4, 3, 4, 4, 5 ); print Dumper (\%h)'

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In reply to array => hash of occurrence indexes by pldanutz

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