As long as you're looking at perl code anyway, backticks should be somewhat familiar...

$ prog.pl `ls file*`
[download]

Of course, this is kinda silly - since you can just run "prog.pl file*" to get the same output. But you may want to do something fancy:

$ prog.pl `grep KEYWORD file* | cut -f1 -d:`
[download]

Mind you, you can't embed backquotes in backquotes. Thankfully, you said "Linux" where I read "bash" over "sh" anyway. So we switch to something like this:

$ prog.pl $(grep $(getkeyword.pl foo) file* | cut -f1 -d:)
[download]

Remember: Unix isn't just a shell for running perl. :-)