Some Help?

If I was a college student, I wouldn't be asking for CS help here, but I'm not a college student, now am I?

I need some help with programming theory. When I make programs, I always give it a UI (as much as you can make a UI in a command prompt) and I ask the user (me, most of the time) to enter stuff.

However, I have a bit of a problem. I cannot make my programs flexible. For example, I am trying to make a program in Perl to go through files that the user specifies and puts the lines it finds in all the files into another file. However, the problem is that I can't make it so there is no file limit. In this kind of situation, i would make all these if () { } elsif() { } blah blah blah statements, but I want the software to be simple. Like, it would just call a subroutine and pass this parameter containing the number of files, and it would just go through the files just like that. However, I'm relatively new to programming, and I can't really do that kind of stuff yet. So I will have to make crazy if() { } elsif() { } statements. But I don't want to do that. And that's why I'm asking you guys for help.

This is the code so far, which is very unfinished.

#!/usr/bin/perl -w
use strict;
use warnings;


print "I will go through files and find lines that\n";
print "occur in all lists. I will put the lines that\n";
print "occur in all lists in another file.\n";
print "How many lists do you have?\n";


my $fileNumbers = ;


&iterating($fileNumbers);


sub iterating
{
    print "What are those $fileNumbers file names?\n";
    print "Type them in one by one.\n";
}