I'm still not clear on exactly what you want to do (part of this might be your code; $sec_arg next to $zero_arg suggests that $sec_arg is actually the *third* arg (where's $first_arg?)

I'll just deal with the logic:

if (FOUND FIRST ARG) {
    #look for second arg; report true if you find it
} else {
    #say "didn't find first arg" and exit
}
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Whether or not you should use a subroutine depends on what the code's doing, and whether you'll need to do similar things in the future a lot; putting it in a sub is a great way to reuse code. Other factors are readability and maintainability: if you want the maintainer to know exactly what's going on at a glance, you could use a sub with a sensible name.

I'm sure there are others 'round here who can expand on these points.

Philosophy can be made out of anything. Or less -- Jerry A. Fodor