SPUG: Slice of HashOfHash

[Yitzchak Scott-Thoennes]

> Given a Hash of a Hash is it possible to make a slice (I presume this
> will be another hash) of a subset of the HoH based upon a value, not the
> key?  For example:
>
> my %HoH = (
>     flintstones =>
>     {
>         husband   => "fred",
>         pal       => "barney",
>     },
>     jetsons =>
>     {
>         husband   => "george",
>         wife      => "jane",
>         "his boy" => "elroy",  # Key quotes needed.
>     },
>     jones =>
>     {
>         husband   => "fred",
>         wife       => "linda",
>     },
>     simpsons =>
>     {
>         husband   => "homer",
>         wife      => "marge",
>         kid       => "bart",
>     }
> );
>
>
> How can I make a new hash  or loop through this HoH where husband "fred"?

How to loop through, and how to take a slice are two different questions. 
If you are looping through, just skip values of the outer hash that don't
match your critera, as others have shown.

If you want a slice, you can make one with an implicit grep loop in the
key list:

    @HoH{ grep { $HoH{$_}{'husband'} eq 'fred' } keys %HoH }

but since you typically want to loop over it, it usually makes sense just
to have one explicit loop over all of %HoH instead.

Sometimes it's most convinient to make an array of the keys of interest
and do various things (loops, slices, etc.) with that instead:

   @have_husband_fred = grep { $HoH{$_}{'husband'} eq 'fred' } keys %HoH;