use warnings;
use strict;

my $bol_priority = qr{ \A priority }xms;  # 'priority' begins a line
my $blank_line   = qr{ \A \s* \z   }xms;

my $saw_priority = '';

while (<DATA>) {

    s{ $bol_priority }{bandwidth}xms if $saw_priority;

    $saw_priority = /$bol_priority/ .. /$blank_line/;

    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

##</code><code>##

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


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