Don't know if it actually solves your problem (untested!):

# creating the packet chunks
my $packet_eth = $ethernet->encode();
my $packet_ip  = $ip->encode();
my $packet = $packet_eth 
   . (0 x (60 - length($packet_eth) - length($packet_ip)))
   . $packet_ip;
[download]

The "x" operator does the right thing when (60 - length($packet_eth) - length($packet_ip)) isn't positive, of course.

30-sec-after update: it would probably pad with character '0' instead of nulls as you want; a quick hack could be:

# creating the packet chunks
my $packet_eth = $ethernet->encode();
my $packet_ip  = $ip->encode();
my $packet = $packet_eth 
   . (pack 'H*', ('00' x (60 - length($packet_eth) - length($packet_ip
+)))) # Note 0 -> 00
   . $packet_ip;
[download]