I need to replace text just after closed </table> tags in PERL

why don't you go through the file line by line, counting all the start/end tags of a table. and if the sum is zero (all starting table tags are closed with an end tag) you do the matching/replacing..

Parsing HTML with regexes is generally a bad idea. It's especially bad in the generalized case of handling arbitrary HTML. However, it typically gets less and less bad the more you can restrict the inputs you are dealing with.

Have you tried using HTML::TreeBuilder or HTML::Parser to parse your HTML? This untested code should do what you want--as far as I can tell from your description, please post sample data and desired results where possible.

# Parse your html
my $t = HTML::TreeBuilder->new_from_content( $html );
$t->eof;
$t->elementify;

my @tables = $t->lookdown( _tag => 'table' );
for my $table ( @tables ) {

    # Skip this table unless it is immediately followed by a br  
    my $br = $table->right; 
    next unless $br->tag eq 'br';

    # Insert the new div
    $br->postinsert('<div>newdiv</div>');
}