# convert to "inverse" hash of arrays (in two steps)
my %hoa;
# first, build a hash of hashes
# (this eliminates duplicates at both levels)
for my $key (keys %hash) {
	for my $val (@{ $hash{$key} }) {
		$hoa{$val}{$key}++;
	}
}
# then, convert the second level of hashes to arrays
for my $vals (values %hoa) {
	$vals = [sort keys %$vals];
}

# debug: inspect the converted data
print Dumper(\%hoa);

# lookup
print join(', ', @{ $hoa{banana} } ), "\n"