sub my_sort {
    print "hello there $a $b\n";
    my $i = $_[0] || 0;
    ...
}
__END__
hello there ARRAY(0x1868244) ARRAY(0x1868298)
Use of uninitialized value in concatenation (.) or string at ...
Use of uninitialized value in concatenation (.) or string at ...
hello there

*** BOOOOM ***