sub foo {
    my $x = 'x' x 10;
    ...
}

##</code><code>##

$ perl -MDevel::Peek -e'my $x = "aaaaaaaaaa"; Dump $x' 2>&1 | grep COW
  FLAGS = (POK,IsCOW,pPOK)
  COW_REFCNT = 1
$ perl -MDevel::Peek -e'my $x = "aaaaa" . "aaaaa"; Dump $x' 2>&1 | grep COW
$ perl -MDevel::Peek -e'my $x = "a" x 10; Dump $x' 2>&1 | grep COW