This works for me on cygwin perl 5.6.2 (Math::BigInt version 0.01): 
#!/usr/local/bin/perl5.6.2
use Math::BigInt;

BEGIN {
    if (!exists &Math::BigInt::as_hex) {
        *Math::BigInt::as_hex = sub {
            my $i = shift;
            my $hex = '';
            do {
                my $mod = $i % 16;
                $hex .= sprintf("%x", $mod);
            } while $i = $i / 16;
            scalar reverse $hex;
        }
    }
}

$z = new Math::BigInt("1234567890"x 3);
print $z->as_hex();

[download]
Slight variations failed completely, though :)

In reply to Re^7: Big Int to hex with 5.6 by ysth
in thread Big Int to hex with 5.6 by ZlR

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