Easiest way to see what it does is to do it with lots of print statements.

use strict;
use warnings;

my $val = 0;
p('Zero');
$val |= 1;
p('Or-ed 1');
$val <<= 1;
p('shift 1');
$val |= 0;
p('Or-ed 0');
$val <<= 1;
p('shift 1');
$val |= 1;
p('Or-ed 1');
$val <<= 1;
p('shift 1');
$val |= 0;
p('Or-ed 0');

#Prints in binary with lowest bit to the right side.
sub p
{
printf '%20s : %4d => ', shift, $val;
print reverse split '', unpack'b*', pack'c', $val;
print "\n";
}
[download]

C:\>perl test.pl
                Zero :    0 => 00000000
             Or-ed 1 :    1 => 00000001
             shift 1 :    2 => 00000010
             Or-ed 0 :    2 => 00000010
             shift 1 :    4 => 00000100
             Or-ed 1 :    5 => 00000101
             shift 1 :   10 => 00001010
             Or-ed 0 :   10 => 00001010
[download]