Modification of a read-only value attempted at TradeExtract.pl line 103 (#1)

    (F) You tried, directly or indirectly, to change the value of a
    constant.  You didn't, of course, try "2 = 1", because the compiler
    catches that.  But an easy way to do the same thing is:

        sub mod { $_[0] = 1 }
        mod(2);

    Another way is to assign to a substr() that's off the end of the string.

Uncaught exception from user code:
    Modification of a read-only value attempted at TradeExtract.pl line 103.