Perhaps this is done only once at compile time?

Yes. In the past, I have confirmed this by generating long lists and taking memory measurements. It can also be seen in the opcode tree

$ perl -MO=Concise -e'@a = (1..3)'
...
-        <1> ex-list lK ->6
3           <0> pushmark s ->4
5           <1> rv2av lKP/1 ->6
4              <$> const[AV ] s ->5
...
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compared with

$ perl -MO=Concise -e'@a = ($x..$y)'
...
-        <1> ex-list lK ->7
3           <0> pushmark s ->4
-           <1> null lKP/1 ->-
6              <1> flop lK ->7
d                 <1> flip[t6] lK ->7
4                    <|> range(other->5)[t5] lK/1 ->c
-                       <1> ex-rv2sv sK/1 ->d
c                          <#> gvsv[*x] s ->d
-                       <1> ex-rv2sv sK/1 ->6
5                          <#> gvsv[*y] s ->6
...
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so the range operator does NOT generate a list

In fact, there is no range operator.

$ perl -MO=Concise -e'for ($x..$y) {}'
...
7        <{> enteriter(next->9 last->c redo->8) lKS ->a
-           <0> ex-pushmark s ->3
-           <1> ex-list lK ->6
3              <0> pushmark s ->4
-              <1> ex-rv2sv sK/1 ->5
4                 <#> gvsv[*x] s ->5
-              <1> ex-rv2sv sK/1 ->-
5                 <#> gvsv[*y] s ->6
6           <#> gv[*_] s ->7
...
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compared with

$ perl -MO=Concise -e'for ((),$x..$y) {}'
...
8        <{> enteriter(next->a last->d redo->9) lK ->b
-           <0> ex-pushmark s ->3
-           <1> ex-list lKM ->7
3              <0> pushmark sM ->4
-              <0> stub lPM ->4
-              <1> null lKM/1 ->-
6                 <1> flop lKM ->7
g                    <1> flip[t4] lK ->7
4                       <|> range(other->5)[t3] lK/1 ->f
-                          <1> ex-rv2sv sK/1 ->g
f                             <#> gvsv[*x] s ->g
-                          <1> ex-rv2sv sK/1 ->6
5                             <#> gvsv[*y] s ->6
7           <#> gv[*_] s ->8
...
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