#... at the start of your code ...
     # notice how exception definition and message format
     # string constant are in two different places and need
     # to be manually coordinated by the programmer.

     use Exception::Class {
       'Exception::Copy::Mine' {
           fields => [qw(from to)];
        }
        # ... lots of other exceptions here ...
     }
     my $MSG_COPY='Could not copy A.txt to B.txt";

     ... later on when you throw the exception ...

     # notice the repetition in the use of exception
     # properties; the repetition is error prone and adds
     # unnecessary extra typing     

     my $sMsg = sprintf($MSG_COPY, 'A.txt', 'B.txt');
     Exception::Copy::Mine->throw(error => $sMsg
                                  , from => 'A.txt'
                                  , to => 'B.txt');

##</code><code>##

    # the declaration puts the message format string and the
    # class declaration together for the programmer, thus
    # resulting in less maintenence work

    declareExceptionClass("Exception::Mine::Copy"
       , "Could not copy %s to %s", [ qw(from, to) ]);

    .... some where else in your code ...

    # there is no need to explicitly call sprintf or
    # repetitively type variable names, nor even remember
    # the order of parameters in the format string or check
    # for undefined values. Both of these will produce
    # the same error message:
    #   "Could not copy A.txt to B.txt"

    die "Exception::Mine:Copy"->new(from =>'A.txt', to=>'B.txt');
    die "Exception::Mine:Copy"->new(to =>'B.txt', from=>'A.txt');

     # and this will politely fill in 'undef' for the
     # property you leave out:
     #    "Could not copy A.txt to <undef>"

     die "Exception::Mine::Copy"->new(from=>'A.txt');