my $length = 3;
for (0..2**$length-1) {
    printf("%0${length}b\n", $_);
}
[download]

As a function:

sub bits {
    my ($length) = @_;
    return map sprintf("%0${length}b\n", $_), 0..2**$length-1;
}
[download]

How long are you talking about?

Length 10 generates ~1,000,000 results
Length 20 generates ~1,000,000,000 results
Length 30 generates ~1,000,000,000,000 results

Update: Fixed placement of curlies.