This appears to do what you expect, although it replaces all octets but the last with '###' (rather than based on the number of digits in the octet):

s/(([0-2]?\d{1,2}\.){3})([0-2]?\d{1,2})/###.###.###.$3/g;
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Code to test regex:

$ perl -le 'foreach my $c ( q{123.45.67.89 123.45.67.98}, qw/ 0.0.0.0 
+10.1.2.3 172.001.002.003 192.168.128.1 255.255.255.255 / ) { my $d = 
+$c; $d =~ s/(([0-2]?\d{1,2}\.){3})([0-2]?\d{1,2})/###.###.###.$3/g; p
+rint $c, q{ -> }, $d; }'
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Test output:

123.45.67.89 123.45.67.98 -> ###.###.###.89 ###.###.###.98
0.0.0.0 -> ###.###.###.0
10.1.2.3 -> ###.###.###.3
172.001.002.003 -> ###.###.###.003
192.168.128.1 -> ###.###.###.1
255.255.255.255 -> ###.###.###.255
[download]

Hope that helps.