from perldoc perlop:

C-style Logical Defined-Or
       Although it has no direct equivalent in C, Perl's "//" operator
+ is
       related to its C-style or.  In fact, it's exactly the same as "
+||",
       except that it tests the left hand side's definedness instead o
+f its
       truth.  Thus, "$a // $b" is similar to "defined($a) || $b" (exc
+ept that
       it returns the value of $a rather than the value of "defined($a
+)") and
       yields the same result as "defined($a) ? $a : $b" (except that 
+the
       ternary-operator form can be used as a lvalue, while "$a // $b"
       cannot).  This is very useful for providing default values for
       variables.  If you actually want to test if at least one of $a 
+and $b
       is defined, use "defined($a // $b)".

       The "||", "//" and "&&" operators return the last value evaluat
+ed
       (unlike C's "||" and "&&", which return 0 or 1). Thus, a reason
+ably
       portable way to find out the home directory might be:

           $home =  $ENV{HOME}
                 // $ENV{LOGDIR}
                 // (getpwuid($<))[7]
                 // die "You're homeless!\n";

       In particular, this means that you shouldn't use this for selec
+ting
       between two aggregates for assignment:

           @a = @b || @c;              # this is wrong
           @a = scalar(@b) || @c;      # really meant this
           @a = @b ? @b : @c;          # this works fine, though

       As more readable alternatives to "&&" and "||" when used for co
+ntrol
       flow, Perl provides the "and" and "or" operators (see below).  
+The
       short-circuit behavior is identical.  The precedence of "and" a
+nd "or"
       is much lower, however, so that you can safely use them after a
+ list
       operator without the need for parentheses:

           unlink "alpha", "beta", "gamma"
                   or gripe(), next LINE;

       With the C-style operators that would have been written like th
+is:

           unlink("alpha", "beta", "gamma")
                   || (gripe(), next LINE);

       Using "or" for assignment is unlikely to do what you want; see 
+below.
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