use HTML::TokeParser::Simple;

my $p = HTML::TokeParser::Simple->new( $filename );

while ( my $token = $p->get_token ) {
   if ( $token->is_tag('img') && !defined $token->get_attr('alt') ) {
      $token->set_attr('alt', $alt_text{ $token->get_attr('src') } );
   }
   print $token->as_is;
}