This code assumes the number comes as an argument to our function. It comes in at 68 (updated to allow for odd-digited credit cards).

sub luhn {
#234567890123456789012345678901234567890123456789012345678901234567
$a=0;$_=reverse pop;s/(.)(.?)/$a+=$1+($2&&(($2*2-1)%9)+1)/ge;1>$a%10 
}
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Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a (from-home) job
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;