How about something like this:

    sub mask_short_to_long1 {

        my @octets = unpack "C4", pack "V", 2 ** $_[0] -1 ;

        @octets = map {unpack "C", pack "b8", sprintf "%08b", $_} @oct
+ets;

        return join '.', @octets;
    }
[download]

Or, this for Perl versions before 5.6.0 (i.e., without "%b"):

    sub mask_short_to_long2 {

        my @octets = unpack "b8b8b8b8", pack "V", 2 ** $_[0] -1 ;

        @octets = map {ord pack "B8", $_} @octets;

        return join '.', @octets;
    }
[download]

Or, a more direct approach:

    sub mask_short_to_long3 {

        my $mask = shift;
        my @octets = (0,0,0,0);

        for (@octets) {
            $_    = $mask > 8 ? 255 : 256 - 2**(8-$mask);
            $mask = $mask > 8 ? $mask -8 : 0;
        }

        join '.', @octets;
    }
[download]

It is a little bit long for a one-liner but since you asked: perl -le'print+join".",map{ord pack"B8",$_}unpack"b8"x4,pack"V",-1+2**pop' num

--
John.