DB<10> @h{1..26} = a..z

  DB<11> use List::Util qw/pairgrep/

  DB<12> x pairgrep { $b =~ /[aeiou]/ } %h
0  15
1  'o'
2  21
3  'u'
4  9
5  'i'
6  5
7  'e'
8  1
9  'a'
  DB<13> use List::Util qw/pairmap/

  DB<14> x pairmap { $b =~ /[aeiou]/ ? [$a => $b] : () } %h
0  ARRAY(0x349eed8)
   0  15
   1  'o'
1  ARRAY(0x349ef20)
   0  21
   1  'u'
2  ARRAY(0x349ef80)
   0  9
   1  'i'
3  ARRAY(0x349efe0)
   0  5
   1  'e'
4  ARRAY(0x349f040)
   0  1
   1  'a'
  DB<15>