Untested: 
open FIN, "dwtest.dat" or die;
my $bytes = do { local $/; <FIN> }; # slurrrrrp
my $bits = unpack "B*", $bytes;
my $count = $bits =~ tr/1/1/;
print "$count\n";

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-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

In reply to Re: counting set bits in binary file by merlyn
in thread counting set bits in binary file by dwhite20899

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