$short = "1234";

$result_1 = $short;
$result_1 .= "_001" unless ( 4 < length $result_1 );

$result 2 = $short;
$result_2 .= "_001 unless ( substr( $result_2, 4, ) );

$result 3 = $short;
$result_3 .= "_001 unless ( index( $result_3, '_', 4) ); 

##</code><code>##

$result_4 = $short . ( index( $short, 4, 1 ) ) 
                     ? "" 
                     : "_001";