$SIG{_ _WARN_ _} = \&SigWarn; # ohne leerzeichen zw _ _
...
sub SigWarn {
    print "-" x 60, "\n";
    print @_;
    print "\nBacktracking:\n";

    my $count = 1;
    while ($count > -1) {
    my ($package, $filename, $line, $sub) = caller($count);
    last unless defined $line;
    printf("%02i %5i %-20s %-20s\n", $count++, $line, $sub, $filename);
    } # while


} # SigWarn