$ perl -MO=Concise -e 'my $x; $x = "$y $z"'
b  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
3     <0> padsv[$x:1,2] vM/LVINTRO ->4
4     <;> nextstate(main 2 -e:1) v:{ ->5
a     <@> stringify[$x:1,2] sK/TARGMY,1 ->b
-        <0> ex-pushmark s ->5
9        <2> concat[t3] sKS/2 ->a
7           <2> concat[t2] sK/2 ->8
-              <1> ex-rv2sv sK/1 ->6
5                 <$> gvsv(*y) s ->6
6              <$> const(PV " ") s ->7
-           <1> ex-rv2sv sK/1 ->9
8              <$> gvsv(*z) s ->9
-e syntax OK
$ perl -MO=Concise -e 'my $x; $x = ""."$y $z"'
c  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
3     <0> padsv[$x:1,2] vM/LVINTRO ->4
4     <;> nextstate(main 2 -e:1) v:{ ->5
b     <2> concat[$x:1,2] sK/TARGMY,2 ->c
5        <$> const(PV "") s ->6
-        <1> ex-stringify sK/1 ->b
-           <0> ex-pushmark s ->6
a           <2> concat[t3] sKS/2 ->b
8              <2> concat[t2] sK/2 ->9
-                 <1> ex-rv2sv sK/1 ->7
6                    <$> gvsv(*y) s ->7
7                 <$> const(PV " ") s ->8
-              <1> ex-rv2sv sK/1 ->a
9                 <$> gvsv(*z) s ->a
-e syntax OK
$ perl -MO=Concise -e 'my $x = "$y $z"'
a  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
9     <2> sassign vKS/2 ->a
-        <1> ex-stringify sK/1 ->8
-           <0> ex-pushmark s ->3
7           <2> concat[t3] sKS/2 ->8
5              <2> concat[t2] sK/2 ->6
-                 <1> ex-rv2sv sK/1 ->4
3                    <$> gvsv(*y) s ->4
4                 <$> const(PV " ") s ->5
-              <1> ex-rv2sv sK/1 ->7
6                 <$> gvsv(*z) s ->7
8        <0> padsv[$x:1,2] sRM*/LVINTRO ->9
-e syntax OK
$ perl -MO=Concise -e 'my $x; $x = $y." ".$z'
c  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
3     <0> padsv[$x:1,2] vM/LVINTRO ->4
4     <;> nextstate(main 2 -e:1) v:{ ->5
b     <2> sassign vKS/2 ->c
9        <2> concat[t3] sKS/2 ->a
7           <2> concat[t2] sK/2 ->8
-              <1> ex-rv2sv sK/1 ->6
5                 <$> gvsv(*y) s ->6
6              <$> const(PV " ") s ->7
-           <1> ex-rv2sv sK/1 ->9
8              <$> gvsv(*z) s ->9
a        <0> padsv[$x:1,2] sRM* ->b
-e syntax OK