Is it possible to use Perl to read SEGY and SEGD files?
Yes, it is possible. Not only that but also it is possible to edit the files and work with then very quickly.
Is it possible to do libraries for Excel with C#?
Yes. It is possible. I recommend using Excel-DNA. With Excel-DNA is possible to create native xll add-ins for Excel using C#
How can I program a C# app for GIS application?
An easy way to do that is using MagWinGIS. It is an Activex that allows adding GIS interface to an app.