Take your pick:

($from, $to) = ();
undef $_ for $from, $to;
map undef $_, $from, $to;
$from = $to = undef;
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A benefit of undef taking one argument is that you don't have to use parenthesis. You can use:

    undef $from, $to = 3 if $from > $to;
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which you couldn't if undef were to take a list. (Well, you could, but then $to wouldn't be 3 afterwards). Compare it to lc, which doesn't take a list either.