Tattletale Variables

Sometimes you might be faced with a huge program that, somewhere, is changing a variable's value to something undesired.

 use Data::Dumper;

 sub some_long_faraway_function {
     my $href = shift;
     # Pretend there's a lot of code here I don't want to sift through
     $href->{bananas} = 'some bad value';
 }

 my $shopping_list = {
     apples => 1,
     pears => 3,
     bananas => 5,
 };
 some_long_faraway_function($shopping_list);
 warn Dumper(\$shopping_list);

Output:

$VAR1 = \{
            'apples' => 1,
            'bananas' => 'some bad value',
            'pears' => 3
        };

You don't know where it's being changed, but you need to find out. Change the variable so that it tells you where it's being changed.

 package TattletaleScalar;
 use Carp qw(cluck);
 require Tie::Scalar;
 our @ISA = qw(Tie::StdScalar);
 sub STORE {
     warn "TATTLETALE variable set to {$_[1]}";
     cluck();
     ${$_[0]} = $_[1];
 }

 package main;

 use Data::Dumper;

 sub some_long_faraway_function {
     my $href = shift;
     # Pretend there's a lot of code here I don't want to sift through
     $href->{bananas} = 'some bad value';
 }

 my $shopping_list = {
     apples => 1,
     pears => 3,
     bananas => 5,
 };
 my $tmp = $shopping_list->{bananas}; # Save current value
 tie $shopping_list->{bananas}, 'TattletaleScalar';
 $shopping_list->{bananas} = $tmp; # Restore saved value
 some_long_faraway_function($shopping_list);
 warn Dumper(\$shopping_list);

Now we can see the stack every time the variable is changed:

TATTLETALE variable set to {5} at example.pl line 7.
at example.pl line 8.
    TattletaleScalar::STORE(TattletaleScalar=SCALAR(0x7fac290d3260), 5) called at example.pl line 29
TATTLETALE variable set to {some bad value} at example.pl line 7.
at example.pl line 8.
    TattletaleScalar::STORE(TattletaleScalar=SCALAR(0x7fac290d3260), "some bad value") called at example.pl line 19
    main::some_long_faraway_function(HASH(0x7fac29026508)) called at example.pl line 30
$VAR1 = \{
            'apples' => 1,
            'pears' => 3,
            'bananas' => 'some bad value'
        };