High Quality Content by WIKIPEDIA articles! XS is an interface through which a Perl program can call a C or C++ language subroutine. The letters stand for eXternal Subroutine, or xsub, where external means external to Perl, i.e. written in some other language. It may be desirable for a Perl program to invoke a C subroutine in order to handle very CPU or memory intensive tasks, to interface with hardware or low-level system facilities, or to make use of existing C subroutine libraries. The Perl interpreter is a C program, so in principle there is no obstacle to calling from Perl to C. However, the XS interface is complex, and using it requires some understanding of the interpreter. The earliest reference on the subject was the perlguts POD, the word guts suggesting the highly technical nature of the material. Subroutine libraries in Perl are called modules, and modules that contain xsubs are called XS modules.