You can put your our $html; outside of the sub a_moduser. or include the sub wanted in a_moduser and prefix $user and $html with my.

sub a_moduser {
        my $user = shift;
        my $html;
[...]
        $html  = '<form action="' . $$config{selfpath} . 'method="post
+">';    
        find(
          sub {
           [...]
           $html .= '<td><input type="radio" name="base_dir" value="' 
+. $_ . '"></td>';
           [...]
         },
      $$config{base_dir});
[...]
}
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