This is 60 characters. I've verified it by comparing the results to the referenced code from http://perl.about.com/library/weekly/aa080600h.htm.

sub luhn {
#        1         2         3         4         5         6
#23456789012345678901234567890123456789012345678901234567890
my$s;$_=reverse@_;s/(.)(.)/$1.$2*2/ge;$s+=$_ for/./g;!chop$s
}
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