Yes, we can. Like that (untested):

...
my $trigger = 'onload';

                 $html .= <<END;
...
                   <td bgcolor="#EEEEEE">@{[ $qry->popup_menu(-id=>'ID
+', -name=>'PS', -size=>10, -values=>$array_ref, -labels=>$hash_ref,  
+-${trigger}=>"fetchData(id,'','','')") ]}</td>
   
                   </tr>
                </table>

END
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The original code did already interpolate a list expression in the heredoc string (@{[ ... ]} was already present). So you just have to replace the fixed text part (onChange) in the above expression (...) with a string variable (here I used $trigger).