$ 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 already
    defined.  It was interpreted as a "" or a 0, but maybe it was a mistake.
    To suppress this warning assign a defined value to your variables.

    To help you figure out what was undefined, perl tells you what operation
    you used the undefined value in.  Note, however, that perl optimizes your
    program and the operation displayed in the warning may not necessarily
    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.


##</code><code>##

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

##</code><code>##

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