Simply the truth.

So if constant folding (+ allocation) and storing the results as part of the code happens at compile time, how would you explain that the (runtime-)prints in the strace output of the following snippet are interleaved with the allocations?

{print STDERR "1"; "x" x 2**20}
{print STDERR "2"; "x" x 2**20}
{print STDERR "3"; "x" x 2**20}
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$ strace -e mmap,munmap,write ./903114.pl
...
write(2, "1", 11)                        = 1
mmap(NULL, 1052672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -
+1, 0) = 0x7fe6d7315000
write(2, "2", 12)                        = 1
mmap(NULL, 1052672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -
+1, 0) = 0x7fe6d5e0f000
write(2, "3", 13)                        = 1
mmap(NULL, 1052672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -
+1, 0) = 0x7fe6d5d0e000
munmap(0x7fe6d5d0e000, 1052672)         = 0
munmap(0x7fe6d5e0f000, 1052672)         = 0
munmap(0x7fe6d7315000, 1052672)         = 0
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Also,

$ perl -MO=Concise,-exec ./903114.pl
...
11 <$> const[PV "x"] s
12 <$> const[IV 1048576] s
13 <2> repeat[t9] vK/2          <---
...
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seems to suggest that the repetition operator is applied at runtime.