The story is there's no general answer. If LDDLFLAGS is overridden on the command line (LDDLFLAGS=-R/yada/yada) the value replaces what would ordinarily be there, because it overrides the value set by the hash passed to WriteMakefile in Makefile.PL. That is, WriteMakefile examines @ARGV for variable=value pairs and takes what it finds as overrides. (Observed behavior; I can't find the source code.)

To extend LDDLFLAGS, one would have to know how Makefile.PL sets it. I don't know of any way to do that except by examining the sources. (Afaik there's no MakeMaker option to print variable values, although I suppose -qp could be passed to make(1) to do that instead.)

Peeking inside Makefile.PL, one can easily see that LDDLFLAGS is set to the contanenation "-L$Sybase/lib" and $Config::Config{lddlflags}. So the hack would be something like: perl Makefile.PL LDDLFLAGS=$(perl -e'use Config; print qq("-L$ENV{SYBASE}/lib $Config{lddlflags} -R/yada")') which I guess is OK.

Extending your suggestion, I added a new --other_dlflags option to Makefile.PL and sent a patch to Michael Peppler. Perhaps 1.10 will be a little easier to use. :-)