# convert
my %hoh = map {
		$_ => { map {$_=>1} @{ $hash{$_} } }
	} keys %hash;

# debug: inspect the converted data
use Data::Dumper;
print Dumper(\%hoh);

# lookup
my $search = "banana";
for my $key (keys %hoh) {
	print "$key\n" if $hoh{$key}{$search};
}