I think I might be misunderstanding the problem because it seems straightforward:

foreach (<DATA>) {
    my ($day)  = /(\d+)d/;
    my ($hour) = /(\d+)h/;
    $_ ||= 0 foreach $day, $hour;
    print "Day: ($day) Hour: ($hour)\n";
}

__DATA__
3d
3d 2h
6h
[download]

The prints:

Day: (3) Hour: (0)
Day: (3) Hour: (2)
Day: (0) Hour: (6)

Cheers,
Ovid

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