Here are two examples:

My Perl program starts another program running. Then it opens a data file, loads information from that file, and feeds it to the program it started running as if the information were being manually typed in by me. This is necessary because I have a program that generates sun and moon location data. But it has to be fed instructions, one at a time. Rather than sit there and do that for hours I simply tell the Perl program to find the instructions in a data file and them enter them for me. The program of interest cannot read files by itself. It can only accept information from the keyboard.

At times I want to be able to do something such as type in the letter "k." One of my Perl programs then recognizes that character and executes a series of instructions that are built into the program. It then essentially tells Windows to ignore the character.

By the way, I have done quite a bit of emission spectroscopy work myself over the years. You mentioned that you were doing some work with laser emission lines. I worked with a wide variety of light sources.