The easiest way that I can think of would be within a match rather than by splitting based upon a match pattern ...

my $ipaddr = "192.168.8.205";
my ($iprefix, $ipstuff) = $ipaddr =~ m/(\d+\.\d+)\.(\d+\.\d+)/;

print $iprefix, " ", $ipstuff, "\n";
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perl -e 's&&rob@cowsnet.com.au&&&split/[@.]/&&s&.com.&_&&&print'