Hi to all who have answered!

Thanks for all your input.
Of course my question was driven by "need for speed".
I tried to minimize the number of loops to go over the buffer (2k, 4k, 8k, or 16k of values)
I extended the benchmark script and put the different solutions all in there.
Below you can see the results.

Three things:
-I was astonished that the unpack_pack_unpack way is still twice as fast as my solution using bitstrings.
-the solution with the borrowed pad to 8 bytes is really nifty! With one single buffer padding,
one can nearly achieve what I wanted first. One unpack only.
-Next surprise: the used
 $_ &= $mask for @values;
does the same as
map { $_ &= $mask) @values;
but is about 25% faster....Must be the useless generation of the resulting array...
(the test with @values = map { $_ &= $mask) @values; gets a rate of only 4000/s!!)


So of course I'll go with the fastest solution, which has the advantage that it includes the
easy case of 8 bytes per value at no cost when I pre-compute the unpack format.

Thanks again,
Axel

se strict;
use warnings;
no warnings 'portable';
use Benchmark 'cmpthese';

my $bytes_per_value = 5;
my $count = 1_000;

my $value = 0xf_dead_beef_4;
my $bin_value = substr (pack ('Q', $value), 0, $bytes_per_value);
my $buffer = $bin_value x $count;

my $fmt = sprintf "(b%d)*", $bytes_per_value << 3;

my $pad_len = 8 - $bytes_per_value;
my $mask = ~ 0 >> $pad_len * 8;
my $pad = "\0" x $pad_len;
my @fmts = ('(Q<)', '(Q<X)', '(Q<X2)', '(Q<X3)');
my $fmt2 = $fmts[$pad_len] . sprintf ("%d", $count);

cmpthese -1, {
    strings => sub {
        my @values = map { oct '0b'.reverse ($_)} unpack ($fmt, $buffe
+r);
        return \@values;
    },
    map_unpack => sub {
        my @values =  map { unpack 'Q<', "$_$pad" } unpack "(a$bytes_p
+er_value)*", $buffer;
        return \@values;
    },
    unpack_pack_unpack => sub {
        my @values = unpack '(Q<)*', pack '(a8)*', unpack "(a$bytes_pe
+r_value)*", $buffer;
    },
    numbers_map => sub {
        my @values = unpack "(QX$pad_len)$count", $buffer.$pad;
        map { $_ &= $mask } @values;
        return \@values;
    },
    numbers_for => sub {
        my @values = unpack $fmt2, $buffer.$pad;
        if ($pad_len) {
             $_ &= $mask for @values;
        }
        return \@values;
    },
};
__END__
[download]

                     Rate strings map_unpack unpack_pack_unpack number
+s_map numbers_for
strings            1707/s      --       -28%               -47%       
+ -69%        -77%
map_unpack         2386/s     40%         --               -26%       
+ -56%        -68%
unpack_pack_unpack 3242/s     90%        36%                 --       
+ -41%        -57%
numbers_map        5462/s    220%       129%                68%       
+   --        -28%
numbers_for        7538/s    342%       216%               133%       
+  38%          --
[download]