=> quotes the lefthand side. Thus, perl is trying to DWIM, and interpreting what you wrote as:

my %modestring = 
  (
  'ROTATE', "a",
  'HOLD'  , "b",
  'FLASH' , "c",
  # etc...
  );
[download]

perl -pe '"I lo*`+$^X$\"$]!$/"=~m%(.*)%s;$_=$1;y^`+*^e v^#$&V"+@( NO CARRIER'