Today, deMize asked in the chatterbox how to convert the number 4335043554366887798866555766 to hexadecimal. One solution is of course to use dc:

dc -e16o4335043554366887798866555766p
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The easiest perl solution is probably to use the Math::BigInt module:

perl -wE 'use Math::BigInt; say Math::BigInt->new("4335043554366887798
+866555766")->as_hex;'
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But here's another, more complicated solution.

perl -wE '
sub hadd { 
        no warnings "uninitialized"; 
        my($a, $b) = @_; my @b = @$b; my $c; 
        for (my $k = 0; @b || $c; $k++) { 
                $c = 16 <= ($$a[$k] += $c + shift @b); $$a[$k] %= 16; 
        } 
} 
my $n = "4335043554366887798866555766"; 
my $h = []; for my $d (split //, $n) { 
        hadd $h, $h; hadd my $g = [], $h; hadd $h, $_ for $h, $h, $g, 
+[$d]; 
} 
say join "", reverse map { sprintf "%x", $_; } @$h;
'
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Update: deleted the unnecessary sub from the code.

Update 2011-03-18: see also Re: Module for 128-bit integer math? for a list of bigint modules.

Update 2012-08-25: see also Re: Zeckendorf representation for another conversion, this time to a more unusual number system.