I suppose I was uneasy about the possibility of the  when (/[ab]/) { ... } tail clause 'getting lost', dependent as it is on a  continue statement in other, possibly distant, clauses, and on the relative positions of those clauses (also a dependency of Neighbour's approach). Taking apl's suggestion, I am more comfortable with something like the following, which encapsulates the tail of each 'tailed' clause within the clause itself:

>perl -wMstrict -lE
"for my $s qw(a b c d) {
   given ($s) {
     my $tail_a_and_b = sub { say qq{ after a or b, was '$_' } };
     when ('a') { say 'a';  $tail_a_and_b->(); }
     when ('b') { say 'b';  $tail_a_and_b->(); }
     when ('d') { say 'd';             }
     default    { say qq{other: '$_'}; }
     }
   }
"
a
 after a or b, was 'a'
b
 after a or b, was 'b'
other: 'c'
d
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