...the expression definitely can be evaluated at compile time.

Depends on how you define "compile time" :)

The constant pragma implies a BEGIN block, which has its own compile and execution time — with the execution happening right after the BEGIN block has been compiled, but before the rest of the code is being parsed and compiled.

In other words,

use constant FOOBAR => "a" x 3;
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is roughly equivalent to

BEGIN { my $x = "a" x 3; *::FOOBAR = sub () { $x } }
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And when you investigate it with B::Concise, you'll see that the opcodes in question have just moved to a different place and execution time, but they're still there:

$ perl -MO=Concise,BEGIN,-main -e'use constant FOOBAR => "a" x 3; prin
+t FOOBAR' | grep -C 3 '"a'
14             <$> const[PV "constant"] sM ->15
15             <$> const[PV "FOOBAR"] sM/BARE ->16
18             <2> repeat[t1] sKM/2 ->19                  # <---
16                <$> const[PV "a"] s ->17                # <---
17                <$> const[IV 3] s ->18                  # <---
19             <$> method_named[PV "import"] ->1a
BEGIN 5:
-e syntax OK
--
4i    <;> nextstate(main 102 -e:1) v:{ ->4j
4l    <@> print vK ->4m
4j       <0> pushmark s ->4k
4k       <$> const[PV "aaa"] s ->4l
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The mechanism to define constants via Constant Functions is rather different from constant folding in the sense of 42+99 being replaced with 141 at compile time.

Consider the following

$ perl -MO=Concise -e'use constant FOOBAR => scalar <STDIN>; print FOO
+BAR'
abc
6  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 102 -e:1) v:{ ->3
5     <@> print vK ->6
3        <0> pushmark s ->4
4        <$> const[PV "abc\n"] s ->5     # <---
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Clearly, the value "abc" isn't part of the code, but entered via the keyboard at runtime (of the respective BEGIN block), yet it can be inlined as a constant in code sections compiled subsequently.