I thought I like it too.

But then I found out that it is too complicated to use those constants.

• They won't interpolate into strings leading to
  

  print "The file '".FILENAME."' cant't be read";
  [download]

  instead of

  print "The file '$FILENAME' can't be read";
  [download]

• I couldn't find out how to (elegantly) use constants in REs. /IDRE=VALRE/ won't work whereas /$IDRE=$VALRE/o isn't a problem