A slight change to the code reveals its purpose:

my $n = 4;
for my $i (1..2**$n-1) {
    my $str = "";    
    for my $j (0..$n-1) {
                $str = (($i & 2**$j) ? '1' : '0') .$str;
    }
    print "$i = $str\n"
}
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This prints:

1 = 0001
2 = 0010
3 = 0011
4 = 0100
5 = 0101
6 = 0110
7 = 0111
8 = 1000
9 = 1001
10 = 1010
11 = 1011
12 = 1100
13 = 1101
14 = 1110
15 = 1111
[download]