You can use a shared library by linking it with your main program.
Before linking the library, install it in your system library directory:
$ cp libsubrs.so /usr/lib
or install it somewhere else and set LD_LIBRARY_PATH:
$ cp libsubrs.so /your/cobol/lib
$ export LD_LIBRARY_PATH=/your/cobol/lib
Then, compile the main program, linking the library as follows:
$ cobc main.cob -L/your/cobol/lib -lsubrs