Your guess is incorrect. It's undocumented and potentially undefined, but the concatenation operator always evaluates its LHS before its RHS.

The key is that the variable is placed on the stack, not its value.

First snippet (simplified):

$ perl -MO=Concise,-exec -E'my $foo = "weird123"; say $foo . do { $foo
+ =~ s/123//; $foo };'
...
6  <;> nextstate(main 49 -e:1) v:%,{,469764096
7  <0> pushmark s
8  <0> padsv[$foo:47,49] s
9  <0> enter s
a  <;> nextstate(main 48 -e:1) v:%,{,469764096
b  <0> padsv[$foo:47,49] sRM
c  <$> const[PV ""] s
d  </> subst(/"123"/) vKS
e  <;> nextstate(main 48 -e:1) v:%,{,469764096
f  <0> padsv[$foo:47,49] s
g  <@> leave sKP
h  <2> concat[t2] sK/2
i  <@> say vK
...
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1. (8) $foo is placed on the stack. (The scalar, not its value.)
2. (d) $foo is changed.
3. (f) $foo is placed on the stack. (The scalar, not its value.)
4. (h) The values on the on the stack ($foo and $foo) are replaced with the result of their concatenation.

Replicating the behaviour with an inspectable stack:

use feature qw( say );
use Data::Alias qw( alias );

sub concat { my $lhs = shift; my $rhs = shift; $lhs . $rhs }

{
   my $foo = "weird123";
   alias push @_, $foo;
   $foo =~ s/123//;
   alias push @_, $foo;
   alias push @_, &concat;
   say shift(@_);
}
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Second snippet (simplified):

$ perl -MO=Concise,-exec -E'my $foo = "weird123"; say $foo . "" . do {
+ $foo =~ s/123//; $foo };'
...
6  <;> nextstate(main 49 -e:1) v:%,{,469764096
7  <0> pushmark s
8  <0> padsv[$foo:47,49] s
9  <$> const[PV ""] s
a  <2> concat[t2] sK/2
b  <0> enter s
c  <;> nextstate(main 48 -e:1) v:%,{,469764096
d  <0> padsv[$foo:47,49] sRM
e  <$> const[PV ""] s
f  </> subst(/"123"/) vKS
g  <;> nextstate(main 48 -e:1) v:%,{,469764096
h  <0> padsv[$foo:47,49] s
i  <@> leave sKP
j  <2> concat[t3] sKS/2
k  <@> say vK
...
[download]

1. (8) $foo is placed on the stack. (The scalar, not its value.)
2. (9) An empty string is placed on the stack.
3. (a) The values on the on the stack ($foo and the new value) are replaced with the result of their concatenation.
4. (f) $foo is changed.
5. (h) $foo is placed on the stack. (The scalar, not its value.)
6. (j) The values on the on the stack (the result of the earlier concatenation and $foo) are replaced with the result of their concatenation.

Replicating the behaviour with an inspectable stack:

use feature qw( say );
use Data::Alias qw( alias );

sub concat { my $lhs = shift; my $rhs = shift; $lhs . $rhs }

{
   my $foo = "weird123";
   alias push @_, $foo;
   alias push @_, "";
   alias push @_, &concat;
   $foo =~ s/123//;
   alias push @_, $foo;
   alias push @_, &concat;
   say shift(@_);
}
[download]