If you write that as, do {this} if $var; then there is no poddibility of an else block.

By skipping an expression when a condition is not met, you can avoid lots of difficult chains of logic.

I guess I'm missing the point. You don't have to have an else segment.

If you want your if() to terminate after executing the truth segment, but you don't want to wrap the entire rest of your script inside of an else segment, you can die or exit at the end of the truth segment, and forgo using an else segment. If you want it to execute the truth segment, and then execute the rest of the script regardless of the outcome of the if() evaluation, then just don't use an 'else' segment.

Only use an else segment if you want a mutually exclusive logical branch in the program's execution.

It is a bad idea to put a last there.

1. If this if statement is inside a loop, you will go out of the loop, which is not what you want;
2. If this is not inside a loop, Perl does not like it;

Actually I sometime intentionally add blank else blocks in my code, simply to tell myself that, "I DID think about whether anything need to be done in the else branch, but really nothing need to be done here, I didn't foget".

To do that, simply leave it blank, or put a ; there.

use strict; 
use warnings;

my $var;

if ($var) {
      print $var;
}  else {
    ;
}
print "Hello World!";
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I'm not usually a fan of putting semantics in comments, but here I think I might make an exception for:

if(&predicate(...)) {
  &foo(...);
} else {
  # do nothing
}
[download]

It's a bit more explicit to some poor maintenance programmer than a lone, apparently useless, semi. Of course, if lone, apparently useless, semis are a coding convention for "do nothing" where you hack, it's okay. :-)

--
Yours in pedantry,
F o x t r o t U n i f o r m
"Anything you put in comments is not tested and easily goes out of date." -- tye

if ($var) {
  #do this...
}
#get on with the rest of the script
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Among the decent answers above, FoxtrotUniform makes a point that's worth expanding on a bit.

Someday in the future, someone (perhaps you) might pick up the code to extend it, and wonder at the logic. The conditional logic you've shown is simple, but conditional logic can get awkward. Leaving an empty else (or elsif) block is a way to signal to the future reader that you've considered that branch of the logic tree, and that there's nothing to do. If the logic is painfully simple, leaving a blank note isn't necessary, an intrudes on readability. But if the logic is messy, adding a short explanatory comment to the empty can save some future reader a lot of puzzling and pondering. It's a matter of finding the right balance.

if ($foo) {
   print LOG "Exit condition $foo reached.\n";
   exit 0;
}
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if ($line =~ /\w+sql=\w+/i) {
   $db = "mysql";
   $db_loc = (split(/=/, $line))[1];
}
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else {
   print DEBUG "Error: Couldn't parse a database out of line ($line)\n
+";
}
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print "Items " . join(', ', @items) . " have been found.\n" if $#items
+ != -1;
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Completely offtopic, but that can be better written as:

print "Items " . join(', ', @items) . " have been found.\n" if @items;
[download]

It's a little easier to read, imho.

Being right, does not endow the right to be rude; politeness costs nothing.
Being unknowing, is not the same as being stupid.
Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence.
Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.