Bit::Vector is your friend, especially to_Bin.
Here's an example, all error checking is up to you;)

use Bit::Vector;
$_ = '1234567891234567';
my $vec = Bit::Vector->new_Dec(64, $_);
print $vec->to_Bin, "\n";

Output:
0000000000000100011000101101010100111100100110111010111100000111
[download]

A side note: I had to quote the number to get Bit::Vector to accept it.