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my @fields;
my $bits = unpack('B*', $input);
...
   my $field = substr($bits, 0, $num_bits, '');
   push @fields, $field;
}

use Config qw( );
use constant UV_BITS => $Config::Config{uvsize} * 8;

die if $num_bits > UV_BITS;
push @fields, pack('B*', substr(("0" x UV_BITS).$field, -UV_BITS));