$text = '1 &plus; 2 <maths> &plus; dfdf</maths>';

for( split '\s', $text ){
  if(/<maths>/.../<\/maths>/){
    next
  }else{
    if(/&plus;/){
      $text =~ s/$_/&thinsp;&plus;&thinsp;/
    }
  }
}

#STDOUT: 1 &thinsp;&plus;&thinsp; 2 <maths> &plus; dfdf</maths>