You can accept user input using CGI's param method (after creating some sort of html form of course). You can find the file using glob or opendir/readdir/closedir or File::Find, and you can display the file perhaps using open to open the file, read or readline to read the file, and print to display the file.

You should also look at taint in perlsec, since there are security issues with accepting user input for use in system commands like open.

If you need more help, you should probably write a bit of code and ask for help with specific things you are having trouble with.