my $handler = do { no strict 'refs'; \&{"handle_fieldcode_$code"} };
...handle expections somehow?...
die if !$handler;
$handler->(@args);

##</code><code>##

my $handler = $lookup{$code};
die if !$handler;
$handler->(@args);

##</code><code>##

my %lookup = map {
   no warnings 'refs';
   $_ => \&{"handle_fieldcode_$_"}
} 'A'..'Z';
...modify %lookup for expections...