I don't see it in the source code, but once I build perl from the source, I see a file ext/DynaLoader/DynaLoader.c which contains its definition:

#ifdef __cplusplus
extern "C" {
#endif
XS_EXTERNAL(boot_DynaLoader); /* prototype to pass -Wmissing-prototype
+s */
XS_EXTERNAL(boot_DynaLoader)
{
#if PERL_VERSION_LE(5, 21, 5)
    dVAR; dXSARGS;
#else
    dVAR; dXSBOOTARGSXSAPIVERCHK;
#endif
#if PERL_VERSION_LE(5, 8, 999) /* PERL_VERSION_LT is 5.33+ */
    char* file = __FILE__;
#else
    const char* file = __FILE__;
#endif

    PERL_UNUSED_VAR(file);

    PERL_UNUSED_VAR(cv); /* -W */
    PERL_UNUSED_VAR(items); /* -W */
#if PERL_VERSION_LE(5, 21, 5)
    XS_VERSION_BOOTCHECK;
#  ifdef XS_APIVERSION_BOOTCHECK
    XS_APIVERSION_BOOTCHECK;
#  endif
#endif

        newXS_deffile("DynaLoader::dl_load_file", XS_DynaLoader_dl_loa
+d_file);
        newXS_deffile("DynaLoader::dl_unload_file", XS_DynaLoader_dl_u
+nload_file);
        newXS_deffile("DynaLoader::dl_find_symbol", XS_DynaLoader_dl_f
+ind_symbol);
        newXS_deffile("DynaLoader::dl_undef_symbols", XS_DynaLoader_dl
+_undef_symbols);
        newXS_deffile("DynaLoader::dl_install_xsub", XS_DynaLoader_dl_
+install_xsub);
        newXS_deffile("DynaLoader::dl_error", XS_DynaLoader_dl_error);
#if XSubPPtmpAAAA
        newXS_deffile("DynaLoader::CLONE", XS_DynaLoader_CLONE);
#endif

    /* Initialisation Section */

#line 171 "DynaLoader.xs"
    (void)dl_private_init(aTHX);

#if XSubPPtmpAAAA
#endif
#line 591 "DynaLoader.c"

    /* End of Initialisation Section */

#if PERL_VERSION_LE(5, 21, 5)
#  if PERL_VERSION_GE(5, 9, 0)
    if (PL_unitcheckav)
        call_list(PL_scopestack_ix, PL_unitcheckav);
#  endif
    XSRETURN_YES;
#else
    Perl_xs_boot_epilog(aTHX_ ax);
#endif
}

#ifdef __cplusplus
}
#endif
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