use strict;
use warnings;

open my $inFH, q{<}, \ <<EOD or die qq{open: <<HEREDOC: $!\n};
Class control
priority 5 
Class voip
priority 30 
Class video
priority 40 


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

{
    local $/ = q{};
    while( <$inFH> )
    {
        my $foundPriority = 0;
        $_ =
           join q{},
           map {
                   unless( m{\bpriority\b} )
                   {
                       ;
                   }
                   elsif( $foundPriority ++ )
                   {
                       s{priority}{bandwidth};
                   }
                   else
                   {
                       ;
                   }
                   $_;
               }
           split m{(\s+)};
        print;
    }
}

close $inFH or die qq{close: <<HEREDOC: $!\n};

##</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