Warning, a curious bug in 5.6.1 with tied variables and interpolation/concatenation, may cause some mental anguish. It seems that a double FETCH is happening, but only when the variable is interpolated at the front of a string that contains trailing characters (or the equivalent, when the variable is the leading term in a concatenation operation):

#!/usr/bin/perl -w
use strict;

{
    package Tie::IncScalar;
    sub TIESCALAR {
        my $class = shift;
        my $self = shift || 0;
        return bless \$self, $class;
        
    }
    sub FETCH {
        my $self = shift;
        $$self++;
    }
    sub STORE {
        my $self = shift;
        $$self = shift;
    }
}

tie my $x, 'Tie::IncScalar';
$x = 0;
print "Case: 1 (good)\n";
print $x for 1..10;
print "\n";
$x = 0;
print "$x" for 1..10;
print "\n\n";

$x = 0;
print "Case: 2 (good)\n";
print " $x" for 1..10;
print "\n";
$x = 0;
print " " . $x for 1..10;
print "\n\n";

$x = 0;
print "Case: 3 (good)\n";
print " $x " for 1..10;
print "\n";
$x = 0;
print " " . $x . " " for 1..10;
print "\n\n";

$x = 0;
print "Case: 4 (yikes)\n";
print "$x " for 1..10;
print "\n"; 
$x = 0;
print $x . " " for 1..10;
print "\n\n";
__END__
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This produces the following output with 5.6.1:

Case: 1 (good)
0123456789
0123456789

Case: 2 (good)
 0 1 2 3 4 5 6 7 8 9
 0 1 2 3 4 5 6 7 8 9
 0 1 2 3 4 5 6 7 8 9

Case: 3 (good)
 0  1  2  3  4  5  6  7  8  9 
 0  1  2  3  4  5  6  7  8  9 
 0  1  2  3  4  5  6  7  8  9 

Case: 4 (yikes)
1 3 5 7 9 11 13 15 17 19 
1 3 5 7 9 11 13 15 17 19
[download]

Case 4 works fine with 5.00503 and 5.6.0. (I noticed this behaviour when playing with Tie::Cycle earlier ... same thing, skips elements in the cycle ... and if you first encounter it while playing with a two value (boolean) cycle, you may feel like you are simply whacked out because you can't see why the cycle isn't cycling :-(

I searched p5p and it appears to have been reported and patched in development versions --- I just wanted to let people here know so they might avoid some confusion if they run into it. A relevant p5p link (from which the patch is linked) is: here