The slow bit is the function call, not the hash lookup which you'd also have to do with the symtab anyway. With a hash, you have better control over what inputs are allowed and how they are dispatched.

With symtab:

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

With hash;

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

with a one-time setup of

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