Definition of empty {} after If() statement in eval{} statement

Question

I am currently attempting to document a Perl script in preparation for converting it to .NET. I have no prior experience in Perl before now, however I was managing to get through it with a lot of Google-fu. I have run into a single line of code that has stopped me as I am unsure of what it does. I've been able to figure out most of it, but I'm missing a piece and I don't know if it's really that important. Here is the line of code:

eval { if(defined $timeEnd && defined $timeStart){}; 1 } or next;

I know that defined is checking the variables $timeEnd and $timeStart to see if they are null/nothing/undef. I also believe that the eval block is being used as a Try/Catch block to trap any exceptions. The line of code is in a foreach loop so I believe the next keyword will continue on with the next iteration of the foreach loop.

The part I'm having difficulty deciphering is the {};1 bit. My understanding is that the ; is a statement separator in Perl and since it's not escaped with a backslash, I have no idea what it is doing there. I also don't know what the {} means. I presume it has something to do with an array, but it would be an empty array and I don't know if it means something special when it is directly after an if() block. Lastly, I no idea what a single integer of 1 means and is doing there at the end of an eval block.

If someone could break that line of code down into individual parts and their definitions, I would greatly appreciate it.

Bonus: If you can give me a .NET conversion, and how each Perl bit relates to it, I will most certainly give you my internet respects. Here's how I would convert it to VB.NET with what I know now:

For each element in xmlList    'This isn't in the Perl code I posted, but it's the `foreach` loop that the code resides in.
    Try
        If Not IsNothing(timeEnd) AND Not IsNothing(timeStart) then 
        End If
    Catch ex as Exception
        Continue For
    End Try
Next

Answer 1

Ignoring elsif and else clasuses, the syntax of an if statement is the following:

if (EXPR) BLOCK

The block is executed if the EXPR evaluates to something true. A block consists of a list of statements in curly braces. The {} in your code is the block of the if statement.

It's perfectly valid for blocks to be empty (to contain a list of zero statements). For example,

while (s/\s//) { }

is an inefficient way of doing

s/\s//g;

The thing is, the condition in the following has no side-effects, so it's quite useless:

if(defined $timeEnd && defined $timeStart){}

It can't even throw an exception!^[1] So

eval { if(defined $timeEnd && defined $timeStart){}; 1 } or next;

is equivalent to

eval { 1 } or next;

which is equivalent to^[2]

1 or next;

which is equivalent to

# Nothing to see here...

---

1. Technically, it can if the variables are magical.

   $ perl -MTie::Scalar -e'
      our @ISA = "Tie::StdScalar";
      tie(my $x, __PACKAGE__);
      sub FETCH { die }
      defined($x)
   '
   Died at -e line 4.
   

   I doubt the intent is to check for this.

2. Technically, it also clears $@.