Constants created by use constant are actually functions, or at least act like them.

The parens aren't needed in the code you gave, but they don't harm anything either.

>perl -MO=Concise -e"use constant BUFFER => 20; if (BUFFER() != 20) {}
+"
3  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 72 -e:1) v ->3
-     <0> ex-const v/4 ->3
-e syntax OK

>perl -MO=Concise -e"use constant BUFFER => 20; if (BUFFER != 20) {}"
3  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 72 -e:1) v ->3
-     <0> ex-const v/4 ->3
-e syntax OK
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They do help in other circumstances, though. If a bareword is expected, then the constant will be treated as a bareword instead of as a constant. EXPR => might be the only problem.

>perl -le"use constant FOO => 'abc'; print %{{ FOO => 'def' }};"
FOOdef

>perl -le"use constant FOO => 'abc'; print %{{ FOO() => 'def' }};"
abcdef
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Update: Interestingly, neither $hash{EXPR} and sub(*)->{EXPR} treat the constant as a bareword. EXPR => might be the only case where it's an issue.