If you're concerned about disc space, though, I'm going to assume that the files are very large, ergo, unlikely to fit in memory... you'd likely want to do something like:

dbmopen %ORIG, "original", 0666 or die;
dbmopen %NOVA, "new", 0666 or die;
# -- race condition start
for my $key (keys %ORIG)
{
 $NOVA{$key} = $ORIG{$key};
}
dbmclose %ORIG;
dbmclose %NOVA;
unlink "original"
link "new", "original"
# race condition end
unlink "new";
[download]

So long as no-one else needs to get at your DBM while you're "shrinking" it, the above will work.