perl -MDevel::Peek -le 'for my $x ([1]->[0]) {Dump $x; $x = pack "I", $x; Dump $x}'         # OK
perl -MDevel::Peek -le 'my $y=1; for my $x ($y) {Dump $x; $x = pack "I", $x; Dump $x}'      # OK
perl -MDevel::Peek -le 'sub one(){1} for my $x (one) {Dump $x; $x = pack "I", $x; Dump $x}' # OK
perl -MDevel::Peek -le 'for my $x (0+1) {Dump $x; $x = pack "I", $x; Dump $x}'              # OK!!!
perl -MDevel::Peek -le 'for my $x ("1"."1") {Dump $x; $x = pack "I", $x; Dump $x}'          # OK

perl -MDevel::Peek -le 'for my $x ("1") {Dump $x; $x = pack "I", $x; Dump $x}'              # Error, read-only
perl -MDevel::Peek -le 'for my $x (1) {Dump $x; $x = pack "I", $x; Dump $x}'                # Error, read-only