A somewhat different approach. If you were using 5.8 the $^N is probably a better choice than $+ though it makes little difference in this case.

#! perl -sw
use strict;

for ('6d', '3h', '4d 3h','6D', '3H', '4D 3H') {
    our ($d,$h) = (0,0);
    print "$d days $h hours\n" if
        m[^
            (?: (\d+)d (?{ $d = $+ || '0' }) )?
            \s*
            (?: (\d+)h (?{ $h = $+ || '0' }) )?
        $]xi;
}
__END__
C:\test>test
6 days 0 hours
0 days 3 hours
4 days 3 hours
[download]