Well, building a useful reverse index in cases where you don't have a 1:1 mapping isn't too bad (although not as elegant):

(given %index)

my %revindex;
for my $revpair (map {
  [ $index{$_}, $_ ]
} keys(%index)) {
  push @{$revindex{$revpair->[0]}}, $revpair->[1];
}
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There are probably simpler ways to do even that, however.