# Lets create a hash that has some similar keys and some unsimilar keys 
%h1 = ( cat => 1, dog=> 2, catalog => 3, doggerel => 4 }

x \%h1
0  HASH(0x7f9cb41aaad0)
   'cat' => 1
   'catalog' => 3
   'dog' => 2
   'doggerel' => 4

# See ALL the keys ...
x keys %h1
0  'cat'
1  'doggerel'
2  'catalog'
3  'dog'

# grep allows us to filter that list
x grep { /cat/ } keys %h1
0  'cat'
1  'catalog'

# You access a single hash element with $h{$key}
# You can access multiple elements, called a slice,
# using an array or list of keys. In this case you have to put 
# a '@'  sigil in front, rather than a '$', to indicate 
#you're expecting multiple values
x @h1{grep { /cat/ } keys %h1}
0  1
1  3

# So 'cat' -like keys have the values, '1' and '3'.
# If you wanted to assign those to the keys in 
# another hash, I would save the set of keys in a temp:

@a = grep { /cat/ } keys %h1
@b{@a} = @h1{@a}

  DB<14> x \%b
0  HASH(0x7f9cb41bd0e8)
   'cat' => 1
   'catalog' => 3