Devel::DollarAt(3) Give magic abilities to $@


use Devel::DollarAt;
eval "0/0";
print $@, $@->backtrace;


Using eval {}, you may catch Perl exceptions almost like you do it with try {} in Java. However there are days when you miss some features of exceptions. The only thing you know about the error that occured is the string $@, which combines the error message and technical data like the line number.

The Devel::DollarAt module gives some functionality to the $@ scalar. Once you say ``use Devel::DollarAt'', the module is active program-wide. If an exception occurs anywhere in any module, $@ will be globally set to an object of class Devel::DollarAt. Apart from performance, this shouldn't be a problem because $@ tries to be downwardly compatible to the normal $@. However using this package in CPAN modules or large software projects is discouraged.


Use this module only for debugging. Don't think of it as an exception framework for Perl or something like that. It just gives magic abilities to $@, that's all.


Returns a Devel::Backtrace object, which lets you inspect the callers of the fatality.
Returns the name of the file in which the error occured.
Returns the file handle which has most recently be read from at the time of the error.
Returns the line number of "$@->inputhandle" (which is $.) at the time of the error.
Returns the number of the line in which the error occured.
Redispatches this exception to the next eval{}.
Returns a list of objects with information about when this exception was redispatched. Each object has got the accessors ``package'', ``filename'' and ``line''. In string context, the objects will look like ``redispatched from FooPackage at\n''.
Returns a string that looks quite like the normal $@, e. g. ``Illegal division by zero at line 42, <> line 13.'' Devel::DollarAt overloads the "" (stringification) operator to this method.


A very simple (and pointless) way to use Devel::DollarAt is this oneliner:

    perl -MDevel::DollarAt -e '0/0'

It bails out with ``Illegal division by zero at -e line 1.'' and an exit status of 1, just like it would have done if you hadn't supplied -MDevel::DollarAt. This is because the magically modified $@ variable gets stringified when perl prints it as exit reason. If you actually want to see the difference, use

    perl -MDevel::DollarAt=frame -e '0/0'

This bails out with ``[[Illegal division by zero at -e line 1.]]'' so you can see that something has happened.


This module requires that no other code tampers with $SIG{__DIE__} or *CORE::GLOBAL::die.

A not widely known feature of Perl is that it can propagate $@. If you call die() without parameters or with an empty string or an undefined value, the error message will be ``Died''. However, if $@ was set to some value before this, the previous error message will be used with ``\t...propagated'' appended:

    perl -e '$@="7"; die"
    7       ...propagated at -e line 1.

Devel::DollarAt emulates this behaviour.

If you use the above example but leave out the double quotes, perl's behaviour is different as of version 5.8.8:

    perl -e '$@=7; die'
    7 at -e line 1.

Devel::DollarAt does not emulate this behaviour:

    perl -MDevel::DollarAt -e '$@=7; die'
    7       ...propagated at -e line 1.

If a previous $@ is propagated, inputhandle and inputline won't work. They won't be interpolated into the stringified $@, either.

If perl comes across syntax errors, $@ appears to be just a string as usual. Apparently $SIG{__DIE__} won't be called for syntax errors.


Christoph Bussenius <[email protected]>

If you use this module, I'll be glad if you drop me a note. You should mention this module's name in the subject of your mails, in order to make sure they won't get lost in all the spam.


This module is in the public domain.

If your country's law does not allow this module being in the public domain or does not include the concept of public domain, you may use the module under the same terms as perl itself.