Yes, a proper parser (HTML::TokeParser::Simple is my personal favourite) should make this a pretty straightforward job. For example, assuming a hash (%alt_text) with the relevant 'src' attribute as the key and the text as the value (as davido suggested), something like this would be what you're looking for:

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;
}
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