The slurp gets problematic for very large files, of course, so here is a version that uses a fixed buffer. It runs at essentially the same speed - but uses a constant amount of memory instead.

#!/opt/bin/perl

use strict;
use warnings;
use Time::HiRes qw (gettimeofday tv_interval);

my $file = shift;

my $start_time = [gettimeofday];

open(FH,$file) or die("Can't open file: $!\n");
binmode FH;

my $bytes  = -s $file;
my $cursor = 0;
my $stride = 4000000;
my $counted_bits = 0;

while (1) {
    my $buffer;
    my $result = read(FH, $buffer, $stride);
    if (0 == $result) { 
        last;
    } elsif (not defined $result) {
        warn("Error reading from $file: $!\n");
        last;
    }
    my $bits = unpack "B*", $buffer;
    $counted_bits += $bits =~ tr/1/1/;
}
close (FH);
print "Bits: $counted_bits\n";

my $elapsed = tv_interval($start_time);
my $rate    = $bytes / $elapsed;
print "Bytes per second: $rate\n";
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