my %foo_cache;
sub foo {
   $foo_cache{join ",", @_} ||= do {
      ... rest of sub goes here
   }
}

##</code><code>##

my %foo_cache;
sub foo {
   @{$foo_cache{join ",", @_} ||= [ do {
      ... rest of sub goes here
   }]}
}

##</code><code>##

my %foo_cache;
sub foo {
   return @$_[0..$#$_] for $foo_cache{join ",", @_} ||= [ do {
      ... rest of sub goes here
   }]
}