#!perl
use warnings;
use strict;
use 5.010;  # required for //

my %h = (a=>undef, b=>2);
my ($x,$y) = (undef, 'why not');

warn "-"x20, __LINE__, "\n";
printf "will warn: x=|%s| y=|%s| ha=|%s| hb=|%s|\n", $x, $y, $h{a}, $h{b};
warn "-"x20, __LINE__, "\n";
printf "will id:   x=|%s| y=|%s| ha=|%s| hb=|%s|\n", $x//"<undef>", $y//"<undef>", $h{a}//"<undef>", $h{b}//"<undef>";
warn "-"x20, __LINE__, "\n";

sub dbg_printf {
    printf @_;
    for my $i ( 0 .. $#_ ) {
        my @ord = qw(th st nd rd th th th th th th);
        warn ">> it was the ${i}" . $ord[$i%10] . " argument to dbg_printf() that was not defined \n" unless defined $_[$i];
    }
}
warn "-"x20, __LINE__, "\n";
dbg_printf("will warn, but expand: x=|%s| y=|%s| ha=|%s| hb=|%s|\n", $x, $y, $h{a}, $h{b});
warn "-"x20, __LINE__, "\n";