if ( ref( $Elements[$1] ) eq "ARRAY" ) {
   push ( @{$Elements[$l]} }, $value );
}
else {
  $Elements[$1] = [$value];
}
[download]

Note that you do not need to test whether you already have an array ref. as Perl will auto-vivify an anonymous array for you.

$ perl -MData::Dumper -e'
> @arr = (1, 2);
> push @{ $arr[2] }, 3, 4;
> push @arr, 5;
> print Data::Dumper->Dump([\ @arr], [q{*arr}]);'
@arr = (
         1,
         2,
         [
           3,
           4
         ],
         5
       );
$
[download]

Note also that samtregar's comments are above yours, you may have your monitor upside down :-)

Cheers,

JohnGG