by adding "()" after each's name

qwurx [shmem] ~ > perl -MO=Concise,-exec -e 'use constant foo=>"5"; $c
+=4+foo;'
1  <0> enter 
2  <;> nextstate(main 70 -e:1) v
3  <$> const[IV 9] s
4  <#> gvsv[*c] s
5  <2> sassign vKS/2
6  <@> leave[1 ref] vKP/REFC
-e syntax OK
[download]

qwurx [shmem] ~ > perl -MO=Concise,-exec -e 'sub foo(){"5"}; $c=4+foo;
+'
1  <0> enter 
2  <;> nextstate(main 2 -e:1) v
3  <$> const[IV 9] s
4  <#> gvsv[*c] s
5  <2> sassign vKS/2
6  <@> leave[1 ref] vKP/REFC
-e syntax OK
[download]

are identical (except for the code of constant.pm loaded in the first), while

qwurx [shmem] ~ > perl -MO=Concise,-exec -e 'sub foo{"5"}; $c=4+foo;'
1  <0> enter 
2  <;> nextstate(main 2 -e:1) v
3  <$> const[IV 4] s
4  <0> pushmark s
5  <#> gv[*foo] s
6  <1> entersub[t3] sKS/TARG,1
7  <2> add[t4] sK/2
8  <#> gvsv[*c] s
9  <2> sassign vKS/2
a  <@> leave[1 ref] vKP/REFC
-e syntax OK
[download]

leaves the sub as a sub since it doesn't have a null prototype list.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}