Well, duh, yeah, I want to cluck. Woops. However, it still gives me a wierd behaviour:

$ 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 a.pl line 11
        main::foo() called at a.pl line 14 (#1)
Use of uninitialized value in print at a.pl line 11.
 at a.pl line 11
        main::foo() called at a.pl line 14
 at a.pl line 11
        main::foo() called at a.pl line 14


done.
[download]

For some reason, I'm getting that warning twice. Note also that I had to add a few extra lines to the callstack function:

sub callstack
{
    &$oldwarn if $oldwarn;
    require Carp;
    delete $SIG{__WARN__};
    goto &Carp::cluck;
    $SIG{__WARN__} = \&callstack;
}
[download]

For some reason, Carp::cluck calls warn, so disabling the warn handler gets rid of the infinite recursion. A bit more digging changes the above to:

sub callstack
{
    &$oldwarn if $oldwarn;
    require Carp;
    print STDERR Carp::longmess(@_);
}
[download]

which solves the problem - the warning message only shows on the screen once.

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