chomp $readline;
       my ($key , $value) = (split /::/, $readline);
       defined $key and $data_ids{$key} = $value;
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Which is the same as saying:

       chomp $readline;
       my ($key , $value) = (split /::/, $readline);
       $data_ids{$key} = $value if $key;
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Which is the same as saying:

       chomp $readline;
       if ( my ($key , $value) = (split /::/, $readline) ) {
         $data_ids{$key} = $value;
       }
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Sorry, but that's wrong. The three code variants have similar, but not identical effects:

The first one stores any defined key (and value) in %data_ids, including the empty string and 0 (zero), but excluding undef. The second one stores only those keys in %data_ids which are true in boolean context. This excludes the empty string, 0, and undef. The third variant stores any key as long as split returns at least one value. This includes the empty string, 0, and undef.

Also, the scope of $key and $value is limited to the if block in the third variant, but not in the first and second variant.

Alexander