Personally, I'd just use a dispatch table, at least as long as your conditions A, B, C and D conveniently map to strings, like plain strings or regular expressions do:

my $foo = get_user_input();

my %actions = (
  A => sub {a;},
  B => sub {b;},
  C => sub {c; b;},
  D => sub {d;},
);

if (not exists $actions{$foo}) {
    $foo = 'A';
};
$actions{$foo}->();
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