Logical AND and OR may call FETCH on tied variable more than once.
Here is an example using Tie::Cycle, which cycles through a list
of values on each access:
#!/usr/bin/perl -w
use strict;
use Tie::Cycle;
tie my $cycle, 'Tie::Cycle', [0,1,2,3];
for(1..7){
print $cycle, " ";
}
# printed thus far: 0 1 2 3 0 1 2
print "\n", '-' x 10, "\n";
# logical OR returns left expression if TRUE otherwise right
# expression:
my ($x,$y) = (0,42);
my $z;
$z = $x || $y;
print "\$z is $z\n"; # prints: $z is 42 (as expected)
$z = $y || $x;
print "\$z is $z\n"; # prints: $z is 42 (as expected)
print '-' x 10,"\n";
# next use of $cycle should get 3:
my $ook = $cycle || 42;
print "\$ook is $ook\n"; # prints: $ook is 0
And, of course, one wouldn't expect any logical expression of the
form (EXPR || 42) to evaluate to 0. In that last nugget, $cycle is
evaluated and found to be 3 which is true but instead of just
returning that value, perl fetches $cycle's value once again. This
only happens when the expression is a bare variable, not when it is
part of any other expression: That is, had I said
my $ook = $cycle + 0 || $z;, then the expression would have returned 3
(ie, 3 + 0). The same (mis)behaviour can be shown with logical AND as
well. In either case, it appears only to be triggered on
a short circuit.
This *seems* unrelated to a similar bug (double evaluation of tied
variables in interpolated strings) I mentioned
in this node.
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