I think that your confusion comes from escaping the pipe, but since it is within a character-class [...] you don't need to escape it so it's the same both ways.

  DB<100> $hello = "\n |\t\r";
 => "\n |\t\r"
 
  DB<101> $hello =~ m/^[ \|\t\r\n]+$/   # matches
 => 1
 
  DB<102> $hello =~ m/^[ |\t\r\n]+$/    # matches too
 => 1
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