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

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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.

In reply to warning: tied vars may evaluate twice in logical expressions by danger

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