$s = 'now is the time for all good etc.;'
$bef = 'before';  $mid = 'middle';  $aft = 'after';

Regexp:   result   (was it expected?)

s/.+/$bef$mid$aft/:  beforemiddleafter   (expected result,
   if no brackets)
s/.+/$bef[$mid$aft/:   (SYNTAX ERROR: "scalar found
   where operator expected"...)
s/.+/[$bef]$mid$aft/:  [before]middleafter    (expected)
s/.+/$bef[$mid]$aft/:  after                  (huh?)
s/.+/$bef$mid[$aft]/:  before                 (whaa?)