Between "omniscient debugging" for Perl and some really wierd "uninitialised" warnings in a largish script, I'm thinking that one possible solution to my problems is to find a way to get tracebacks to warnings, even those generated by the compiler. As I'm writing this up, it dawns on me that this is exactly what $SIG{__WARN__} is for. Conveniently, perlvar has pretty much exactly this example. So, I try it out. First, the module, 'T.pm': 

package T;

use strict;

my $oldwarn;
sub import
{
    $oldwarn = $SIG{__WARN__};
    $SIG{__WARN__} = \&callstack;
}

sub callstack
{
    &$oldwarn if $oldwarn;
    require Carp;
    Carp::confess(@_);
}

1;

[download]
And then the calling code: 
#! perl -w

use strict;
use diagnostics;

use T;

sub foo
{
    my $x;
    print $x;
}

foo();

print "\n\ndone.\n\n";

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What I expect is: 
$ perl a.pl
Use of uninitialized value in print at a.pl line 11 (#1)
    (W uninitialized) An undefined value was used as if it were alread
+y
    defined.  It was interpreted as a "" or a 0, but maybe it was a mi
+stake.
    To suppress this warning assign a defined value to your variables.

    To help you figure out what was undefined, perl tells you what ope
+ration
    you used the undefined value in.  Note, however, that perl optimiz
+es your
    program and the operation displayed in the warning may not necessa
+rily
    appear literally in your program.  For example, "that $foo" is
    usually optimized into "that " . $foo, and the warning will refer 
+to
    the concatenation (.) operator, even though there is no . in your
    program.

Use of uninitialized value in print at a.pl line 11.
 at T.pm line 16
        T::callstack('Use of uninitialized value in print at a.pl line
+ 11.\x{a}') called at a.pl line 11
        main::foo() called at a.pl line 14 (#1)

done.

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(I'd rather get rid of T::callstack here somehow, but I can live with that for the purposes of finding the context in which my warnings are coming up.) What I actually get is this: 
$ perl a.pl
Use of uninitialized value in print at a.pl line 11 (#1)
    (W uninitialized) An undefined value was used as if it were alread
+y
    defined.  It was interpreted as a "" or a 0, but maybe it was a mi
+stake.
    To suppress this warning assign a defined value to your variables.

    To help you figure out what was undefined, perl tells you what ope
+ration
    you used the undefined value in.  Note, however, that perl optimiz
+es your
    program and the operation displayed in the warning may not necessa
+rily
    appear literally in your program.  For example, "that $foo" is
    usually optimized into "that " . $foo, and the warning will refer 
+to
    the concatenation (.) operator, even though there is no . in your
    program.

Use of uninitialized value in print at a.pl line 11.
 at T.pm line 16
        T::callstack('Use of uninitialized value in print at a.pl line
+ 11.\x{a}') called at a.pl line 11
        main::foo() called at a.pl line 14 (#1)
Uncaught exception from user code:
        Use of uninitialized value in print at a.pl line 11.
 at T.pm line 16
        T::callstack('Use of uninitialized value in print at a.pl line
+ 11.\x{a}') called at a.pl line 11
        main::foo() called at a.pl line 14
 at T.pm line 16
        T::callstack('Use of uninitialized value in print at a.pl line
+ 11.\x{a}') called at a.pl line 11
        main::foo() called at a.pl line 14

[download]
Note that the "done" line is missing. I'm not sure what is causing the "uncaught exception" printout, which I definitely would like to avoid - I'd like the behaviour of the perl code to be otherwise the same, and if I remove the "use T" line from the original script, the warning is triggered, and then the script continues to completion, so I'd like to allow the script to complete even with T.

(No, these are not the final package/file names ;->)

Any direction on this would be appreciated.

In reply to Automatic stack tracebacks in warnings? by Tanktalus

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