Close, but you need the /m modifier (See m). This makes ^ match the beginning of the line and $ match the end of the line anywhere within a single string. Note that you'll have to have the whole file as a string to use this. Also, you'll need to match the whole of each line. So a solution might look like this:

#!/usr/bin/perl
use strict;
use warnings;

my $filename = shift or die "Need a filename";
my $text;

SLURP:{
            local $/;
              open my $fh, '<', $filename
                      or die "Error opening $filename: $!";
                $text = <$fh>;
                  close $fh;
      }

$text =~ s/(^fixed.*)    #match fixed-address
           \n            #followed by newline
           (^hardware.*) #followed by hardware
          /
           $2\n$1 #and switch them (remember newline!)
          /mgx;  #m for multiline, g for multiple matches

print $text;
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