Oops, I guess BigInt wants a string, not a number.

my $bignum = Math::BigInt->new('9007199254740991');

print $bignum->as_bin(),"\n";

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use bigint;

my $bignum = 9007199254740991;
print $bignum->as_bin(),"\n";
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#Correct results:

0b11111111111111111111111111111111111111111111111111111
0b11111111111111111111111111111111111111111111111111111
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