With your first method

my $next_version_key = $ordered_list->[($i+1)]->[0] 
     if( $ordered_list->[($i+1)] );
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The if is evaluated first, comes out to false, and the $ordered_list->[($i+1)]->[0] is never evaluated.

With your second method, the $ordered_list->[($i+1)]->[0] is evaluated first. Even if this value does not exist, it is autovivified.