ATduck is a Hayes-compatible modem emulator written in Perl. It can be used in conjunction with SLiRP to provide internet access to older software or computers that only supports a modem.
ATduck supports the virtual serial ports of popular emulators, including VMware, QEMU, VirtualBox, Bochs, BasiliskII, and DOSBox. It also supports PTYs, so it can look like a serial port to Linux software. It might also work over a physical serial port, but I have no way to test that.
ATduck can be used on Linux and on Cygwin+Windows. See the manual page for more information about running ATduck.