Point 1: "local" is a "rare duck" - it happens seldom. 
         Almost always "my" is better. local is an artifact
         of previous Perl versions, but it still has uses.
Point 2: I figure that setting $_ as an lvalue is a bad idea. 
         That is motivated the fact that Perl can and will
         do this by itself. 
Point 3: I have no problem with ternary expressions. But I
         I do have a problem with an ":" without a "blah;"
Point 4: If we have a complex decision with 4 values, probably
         we can come up with a name for that decision and
         I'd name a sub{} with that name.
Point 5: If the thing we are searching is small and performance
         is not an issue, I would omit "last" and "next"
         in the interest of simplicity.
         
So, this is one way to do this..

my $foo = decide($value);

sub decide
{
   my $value = shift;
   my $decision = 'somethin';
   
   foreach ($value)  #sets $_ to $value!
   {
      $decision = 'wanna'   if (/aaa/);
      $decision = 'be'      if (/bbb/);
      $decision = 'startin' if (/ccc/);
   }
   return ($decision );
}