EDIT: now I get it, the sub returns the evaluation from the unless-condition in the first line.

... WTF ...

You like obfuscation, don't you?

> are there any conditions when Perl would NOT print "some string" in last two lines of code

sure if you set $R to anything false ( EDIT:... it will start from new, otherwise it will print the new value.)

if you wanna avoid that anyone ever unintentionally changes $R, then put the whole closure construct into braces and $R is out of scope for any other code.

{
  my $R;

  sub mySub {
      unless ($R) {
          print "CONDITION WORKS\n";
          $R = 'some string';
      }
  }
}
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