my @tokens = /
  \\. # any backslashed character
  | \[ .*? \] # bracketed string (the one with magical powers)
  | [^\\\[\]]+ # any sequence of non-specials
  | . # catch-all (to inchworm past broken constructs like mismatched brackets or backslash at end of string)
  /xgs;
for (@tokens) {
  if (/^[(.*)\]$/s) { # it's special
    $_ = process($1); # replace with its expansion
  } elsif (/^\\(.)/s) { # it's escaped
    $_ = $1;
  }
}
my $result = join "", @tokens; # if that's what you want