++. You can eliminate reverse and shorten it to 55.

sub luhn {
#        1         2         3         4         5     
#234567890123456789012345678901234567890123456789012345
my$s;$_=pop;~s/(.)(.)/$2.$1*2/ge;$s+=$_ for/./g;!chop$s
}
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and, if you don't care for strict-ness, shave another 5 chars, by eliminating my$s;

/prakash