Here's some code to do some of that:

my $parser = HTML::TokeParser::Simple->new(string => $html);
    
my $new_html;
  
while ( my $token = $parser->get_token ) {   
    for ( 'src', 'href' ) {
        my $attr = $_;
            
        my $value;
        next unless $value = $token->get_attr($attr);
            
        next unless $value =~ /\.(gif|jpe?g|png|swf)$/;
            
        $value =~ s/\/([\.[:word:]\-]+?)$/$new_url$1/;
                        
        $token->set_attr($attr,$value);
    }
    $new_html .= $token->as_is;
}
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Then your result is in $new_html. Of course, this won't handle everything, since you could have references to images, etc in Javascript, for example.