Function look

static/editor.md/lib/codemirror/mode/perl/perl.js:804–806

(stream, c)

// it's like "peek", but need for look-ahead or look-behind if index < 0
function look(stream, c){
  return stream.string.charAt(stream.pos+(c||0));
}

// return a part of prefix of current stream from current position
function prefix(stream, c){