I think that you are going about this in a too C-ish manner. Whenever I encounter a problem where Perl seems to be getting in the way, I tend to find that I'm using the wrong method and by looking around, there is a perlish method or function that addresses the problem.

In this case, you need vec.

#! perl -slw
use strict;

sub xformBits {
    my( $in ) = shift;
    my $out = chr(0) x length( $in );
    vec($out,$_,1) = vec($in,7-int($_/8)+8*(7-$_%8),1) for 0..63;
    return $out;
}

my $example = 'ABCDEFGH';

print "Input: $example";
print join ' ', split'', unpack 'B8', $_ for split '', $example;

my $xformed = xformBits $example;

print "\nOutput: $xformed";
print join ' ', split'', unpack 'B8', $_ for split '', $xformed;

__END__
[12:12:24.25] P:\test>414344
Input: ABCDEFGH
0 1 0 0 0 0 0 1
0 1 0 0 0 0 1 0
0 1 0 0 0 0 1 1
0 1 0 0 0 1 0 0
0 1 0 0 0 1 0 1
0 1 0 0 0 1 1 0
0 1 0 0 0 1 1 1
0 1 0 0 1 0 0 0

Output:     ??f¬
0 0 0 0 0 0 0 0
1 1 1 1 1 1 1 1
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 1
0 0 0 1 1 1 1 0
0 1 1 0 0 1 1 0
1 0 1 0 1 0 1 0

[12:12:49.51] P:\test>
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