You don't have to, but it can be done. It basically lets the parser know that if it encounters those things in a bareword context to treat them as sub invocations rather than bareword strings. That lets you use them without parens before the actual subroutine declaration later in the source.

use strict;
sub foo;

## This line will not cause an error . . .
foo "ABC";

## . . . but this one does
bar "DEF";

sub foo { print @_ }
sub bar { print @_ }
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