%h = (blonk => 2, foo => 3, squink => 5, bar => 8);
    %subset = %h{'foo', 'bar'}; # key/value hash slice
    # %subset is now (foo => 3, bar => 8)

use  5.14.0;
my   %h = (blonk => 2, foo => 3, squink => 5, bar => 8, book => 220);
my   %subset; 
my   $slice =<<END;

     foo
     |
     bar
     |
     book

END

map  $subset{$_} = $h{$_},
grep m/$slice/ox,
keys %h;

print %subset;

use  5.14.0;

my $rows = [
             { username => 'Peter',
               group    => 'wheel'
             },

             { dog      => 'REXX',
               comment  => 'terrible'
             }
           ];

my %subset;
my $c;
my $slice=<<END;

     username
     |
     group
     |
     comment
END

map $subset{$_}=$c->{$_}, grep /$slice/ox, keys %{$c = $_} for @$rows;

say %subset;