How about something like

sub foo {
  ($foo_cache{...} ||=
     do {
       my @result = do {
         ... rest of sub goes here
       }
       sub { @result[0..$#result] };
     };
   )->();
}
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Wait...if you just want to fix scalar context,

my %foo_cache;
sub foo {
   my $index = join ",", @_;
   $foo_cache{$index} ||= do { .... };
   wantarray ?
   @{$foo_cache{$index}
   :
   $foo_cache{$index}[-1];
}
[download]

Would there be more to a list keyword than this?

sub list {
  wantarray ? @_ : $_[-1];
}
[download]