my %requiredby;
for my $key (keys $dependencies) {
	for my $dep (@{$dependencies{$key}}) {
		push @{$requiredby{$dep}}, $key;
	}
}

# Or alternately, using a statement modifier to be shorter
my %requiredby;
for my $key (keys %dependencies) {
	push @{$requiredby{$_}}, $key for @{$dependencies{$key}};
}