You are re-initializing $obj for every call to find_by_tag_name. You need to change your code to something like below:

sub convert {
    my $html = shift;
    
    my $t = HTML::TreeBuilder->new();
    $t->parse($html);
    $t->eof;
    
    my $obj;
       $obj .= DumpTable( $_ ), $/, $/ 
    for $t->find_by_tag_name('table') ;
    return($obj);
}
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