Removed some whitespace, added default variable and placed the code in a subroutine.

#!/usr/bin/perl -w

if (luhn()==0) { print "valid\n"; } else { print "not valid\n"; }

sub luhn
{
          1         2         3         4         5         6         
+7         8         9         10        11        12 
0123456789012345678901234567890123456789012345678901234567890123456789
+01234567890123456789012345678901234567890123456789012
$n='4564123800603607';$o=$_=reverse$n;$o=~s/.(.)/$1*2/ge;s/(.)./$1/ge;
+$o=~s/(.)/$x+=$1/ge;s/(.)/$y+=$1/ge;return($x+$y)%10;
}
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