Closures are not relevant here:

#for (1..2) {
#   foreach my $x (0) { print $x++; }  # Error!
#}

for (1..2) {
   foreach my $x (0..1) { print $x++; }  # 01, 01
}

for (1..2) {
   foreach my $x (0..1,5..6) { print $x++; }  # 0156, 1267
}
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I'm not sure why this only happens if you combine several ranges.

Actually, it also applies with just a single range:

for (1..2) {
   print map { $_++ } 0..1;  # 01, 12
}

for (1..2) {
   print map { $_++ } 0..1,5..6;  # 0156, 1267
}
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Keep in mind that foreach (expr1..expr2) is optimized into a counting loop, so $x is not an alias to a constant list in the case of foreach my $x (0..1).

---

You've apparently found an optimization. If you turn 0..1,5..6 into a non-constant expression, the expression rebuilds the list every time:

for (1..2) {
   foreach my $x (map $_, 0..1,5..6) { print $x++; }  # 0156, 0156
}
[download]

Another fix is:

for (1..2) {
   foreach (0..1,5..6) { my $x = $_; print $x++; }  # 0156, 0156
}
[download]

Personally I'd consider the fact that lists generated from ranges aren't read-only a bug.

Aye. Lists deemed constant for optimization purposes should have its values flagged read-only.

Update: Added a bit to make my explanations easier to follow. Adjusted my code to match the changes the OP did in his "Update 2".