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'
};
