substr($string, index($string, '<tag>'), rindex($string, '</tag>')) =~ s/\n(?!$)/<br>/g;

##</code><code>##

substr($string, index($string, '<tag>'), rindex($string, '</tag>') - index($string, '<tag>')) =~ s/\n/<br>/g;

##</code><code>##

my $st = index($string, '<tag>');
my $en = rindex($string, '</tag>') - $st;

substr($string, $st, $en) =~ s/\n/<br>/g;