Wait, there's more, we can also adapt Re: Opposite of strtol? like this.

This goes to the file Tohex.xs:

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

#include <gmp.h>

MODULE = Math::Tohex  PACKAGE = Math::Tohex

SV *
tohex(num)
        char *num
    CODE:
        char *out;
        ST(0) = sv_newmortal();
        mpz_t big;
        mpz_init_set_str(big, num, 10);
        Newx(out, 4 + mpz_sizeinbase(big, 16), char);
        mpz_get_str(out, 16, big);
        sv_setpv(ST(0), out);
        mpz_clear(big);
        Safefree(out);

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This goes to the file lib/Math/Tohex.pm:

package Math::Tohex;

require Exporter;
require DynaLoader;
our $VERSION = "1.00";
our @ISA = (Exporter::, DynaLoader::);
our @EXPORT = "tohex";

bootstrap Math::Tohex::;

1;
__END__
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And this goes to Makefile.PL:

use ExtUtils::MakeMaker;

WriteMakefile(
    NAME => "Math::Tohex",
    VERSION_FROM => "lib/Math/Tohex.pm",
    LIBS => ["-lgmp"],
);
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Now compile:

perl Makefile.PL && make
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and run:

perl -wE 'use Math::BigInt; say Math::BigInt->new("4335043554366887798
+866555766")->as_hex;'
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and hope for no stupid mistakes leading to memory corruption.