And this solution works always (it does no longer depend on Class voip coming right after the first priority): 
use strict;
use warnings;

while (<DATA>) {
    if (((/^priority/ .. /^\s*$/) || 0) > 1) {
        s/priority/bandwidth/;
    }
    print;
}


__DATA__
Class control
priority 5
Class voip
priority 30
Class video
priority 40


Class control
priority 10
Class voip
priority 25
Class video
priority 45

[download]
It takes advantage of the fact that the return value of the .. operator in scalar context is 1 the first time the start condition is met.

CountZero

A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James

In reply to Re: Replacing duplicate string by CountZero
in thread Replacing duplicate string by ddrew78

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