use Foo qw(usebytes);

##</code><code>##

use Foo;

##</code><code>##

package Foo;
require bytes; # make sure the module is loaded without (un)import

$bytes = 0;    # flag: whether bytes should be enforced
$evalled = 0;  # flag: whether rest of source has been compiled

sub import {
    if ($evalled++) {                   # we have evalled the rest before
        die qq{Can only have one setting of "use bytes" per run of Perl\n}
         if $bytes != ($_[1] eq "usebytes"); # different setting from before
    } else {                            # first time around
        $bytes = ($_[1] eq "usebytes"); # should bytes be enforced?
        local $/;                       # enable slurp mode
        eval <DATA>;                    # compile rest of source (after __DATA__)
        die $@ if $@;                   # die now if an error occurred
    }
};
1;                              # in order for require of Foo.pm to be successful
__DATA__                        # actual source of module starts here
BEGIN {bytes->import if $bytes} # activate bytes if flag set
sub new { bless {},shift }      # example routine