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.

In reply to Re: dec2bin for more than 32 bits by jsprat
in thread Problem with integer to decimal conversion by ruben

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