use strict ;
use warnings ;
use Data::Dumper ;

my $source = {
    f1 => 'garbage',
    f2 => 'more garbage',
    f3 => 'important data',
    f4 => {
        this => 'sub hash',
        is   => 'garbage'
    },
    f5 => {
        f6 => 'more important data',
        f7 => {
            more => 'garbage',
            f8   => 'important data',
        },
        f9 => 'garbage',
    },
    f10 => [ 'important', 'data' ],
    f11 => [ 'more', 'garbage' ]
};

my $filter = {
    f3 => 1,
    f5 => {
        f6 => 1,
        f7 => {
            f8 => 1
        }
    },
    f10 => 1
};

sub sliceTheWholeDarnThing {
	my $s = $_[0] ;
	my $f = $_[1] ;
	my $n = {} ;
	my @keysToGet = keys %{ $f } ;
	@{ $n }{ @keysToGet } = @{ $s }{ @keysToGet } ;
	foreach ( keys %{ $n } ) {
		if ( ref $n->{ $_ } eq 'HASH' ) { 
			$n->{ $_ } = sliceTheWholeDarnThing( $s->{ $_ }, $f->{ $_ } ) ;
		}
	}
	return $n ;
}

my $newHash = sliceTheWholeDarnThing( $source, $filter ) ;

print Dumper( $newHash ) ;