But if $bar is still in scope after the if, how is this any different from just declaring it right before the block ?

In your example you use the extra braces to limit the scope of your variable to prevent it from being a global. If your "preferred" code would be allowed, where do you think $bar should go out of scope? i.e. :

if ($foo) {
   my $bar = "true";
}
print $bar;
print "again" . $bar; #-- does this still work
<cut 500 lines>
print "and again" . $bar; #-- what about this?
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If all those prints work, your code is essentially the same as when you just declare $bar before the if block. If not, at what point does $bar go out of scope ?

Another point: what happens in your code if the condition is false? Would you want a runtime error (because $bar is not declared in that case) ? Or are you thinking more of declaring $bar in both the if and else branch?

There's one possible case where I could see your suggestion being somewhat useful, namely something like this:

my $bar = "hello"; #-- global

sub mysub {
  if (condition) {
    my $bar = "goodbye";
  }
  print $bar;
}
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i.e. where the sub-local variable would obscure an already-existing global. But this is both ugly and bad design.

So to summarize, my main question to you is: when would $bar go out of scope in the code you propose