If I don't setup the DynaLoader like this I get errors like this (IRL, not in the sample):

Can't load module Encode, dynamic loading not available in this perl.
  (You may need to build a new perl executable which either supports
  dynamic loading or has the Encode module statically linked into it.)
 at Encode.pm line 13.
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Finally got it all up and running.

One of the additional steps that were needed was to rename the so-name of the library in all .so files.

For this I used the patchelf tool:

    cp -pL ${PERLLIB}/libperl.so ${DEST}/${PERLSO}
    cp -pL ${PERLLIB}/libpthread.so.0 ${DEST}/
    patchelf --set-soname perl530.so ${DEST}/${PERLSO}
    find ${DEST} -name '*.so' -exec patchelf --replace-needed libperl.
+so.5.30 ${PERLSO} {} \;
[download]

The resultant, and working, program is now:

#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
#include <EXTERN.h>
#include <perl.h>

void *handle;            /* for dl */

#ifndef PERLSO
#  define PERLSO "perl530.so"
#endif

static pTHX;

void xs_init(pTHX);

int main( int argc, char **argv, char **env ) {

  /* Assuming the program binary   /foo/bar/blech */
  char selfpath[PATH_MAX];    /* /foo/bar */
  char scriptname[PATH_MAX];    /* blech */
  char dllpath[PATH_MAX];    /* /foo/bar/PERLSO */
  memset (selfpath,   0, PATH_MAX);
  memset (scriptname, 0, PATH_MAX);
  memset (dllpath,    0, PATH_MAX);

  if ( readlink ("/proc/self/exe", selfpath, PATH_MAX-1 ) > 0 ) {
    char *p = rindex( selfpath, '/' );
    if ( p ) {
      p++;
      strcpy( scriptname, p );
      *p = 0;
    }
    else
      strcpy( scriptname, selfpath );
#ifdef DEBUG
    fprintf( stderr, "selfpath:   %s\n", selfpath );
    fprintf( stderr, "scriptname: %s\n", scriptname );
#endif
  }
  else {
    strncpy( selfpath, argv[0], PATH_MAX-1 );
    char *p = rindex( selfpath, '/' );
    if ( p ) {
      p++;
      strcpy( scriptname, p );
      *p = 0;
    }
    else {
      p = getcwd( selfpath, PATH_MAX-1 );
      strcat( selfpath, "/" );
      strncpy( scriptname, argv[0], PATH_MAX-1 );
    }
#ifdef DEBUG
    fprintf( stderr, "cwdpath:    %s\n", selfpath );
    fprintf( stderr, "scriptname: %s\n", scriptname );
#endif
  }

  /* Open shared lib. */
  strcpy( dllpath, selfpath );
  strcat( dllpath, PERLSO );

#ifdef DEBUG
  fprintf( stderr, "dllpath:    %s\n", dllpath );
#endif

  handle = dlopen( dllpath, RTLD_LAZY );
  if ( !handle ) {
    fprintf( stderr, "%s\n", dlerror() );
    exit(EXIT_FAILURE);
  }

  /* Start perl environment. */
  //PERL_SYS_INIT3( &argc, &argv, &env );
  void (*Perl_sys_init3)( int*, char***, char*** );
  Perl_sys_init3 = (void(*)(int*, char***, char***)) dlsym(handle, "Pe
+rl_sys_init3");
  (*Perl_sys_init3)(&argc, &argv, &env);

  /* Create a perl interpreter. */
  tTHX (*perl_alloc)();
  perl_alloc = (tTHX(*)()) dlsym(handle, "perl_alloc");
  my_perl = (*perl_alloc)();

  /* perl_construct */
  void (*perl_construct)(pTHX);
  perl_construct = (void(*)(pTHX)) dlsym(handle, "perl_construct");
  (*perl_construct)(aTHX);

  /* perl_parse */
  void (*perl_parse)(pTHX, void*, int, char**, char**);
  perl_parse = (void(*)(pTHX, void*, int, char**, char**)) dlsym(handl
+e, "perl_parse");

  /* If we're "perl", behave like perl. */
  if ( !strncmp( scriptname, "perl", 4 ) )
    (*perl_parse)( aTHX, xs_init, argc, argv, env );

  else {

    /* Insert script name in argv. */
    char scriptpath[PATH_MAX];    /* /foo/bar/SCRIPTPREFIXblech.pl */
    strcpy( scriptpath, selfpath );
#ifdef SCRIPTPREFIX
    strcat( scriptpath, "script/" );
#endif
    strcat( scriptpath, scriptname );
    strcat( scriptpath, ".pl" );

    /* To get @INC right we execute it as a -E script. */
    char cmd[2*PATH_MAX+100];
    sprintf( cmd, "@INC=(q{%slib});do q{%s};", selfpath, scriptpath );
#ifdef DEBUG
    fprintf( stderr, "scriptpath: %s\n", scriptpath );
    fprintf( stderr, "cmd:        %s\n", cmd );
#endif

#   define EXTRA_ARGS 3    
    char **ourargv = (char **)calloc( argc+1+EXTRA_ARGS, sizeof(char**
+) );
    ourargv[0] = argv[0];
    ourargv[1] = "-E";
    ourargv[2] = cmd;
    ourargv[3] = "--";
    for ( int i=1; i<=argc; ++i ) {
      ourargv[i+EXTRA_ARGS] = argv[i];
    }
    (*perl_parse)(aTHX, xs_init, argc+EXTRA_ARGS, ourargv, env);
  }

  /* Set @INC to just our stuff. But it's too late. */
  //  char cmd[PATH_MAX+100];
  //  sprintf( cmd, "@INC = (q{%slib});", selfpath );
  //  SV* (*eval_pv)(pTHX, const char*, I32);
  //  eval_pv = (SV* (*)(pTHX, const char*, I32)) dlsym( handle, "Perl
+_eval_pv" );
  //  (*eval_pv)( aTHX, cmd, TRUE );

  /* Run... */
  int (*perl_run)(pTHX);
  perl_run = (int(*)(pTHX)) dlsym(handle, "perl_run");
  int result = (*perl_run)(aTHX);

  /* Cleanup. */
  void(*perl_destruct)(pTHX);
  perl_destruct = (void(*)(pTHX)) dlsym(handle, "perl_destruct");
  (*perl_destruct)(aTHX);
  void(*perl_free)(pTHX);
  perl_free = (void(*)(pTHX)) dlsym(handle, "perl_free");
  (*perl_free)(aTHX);

  return result;
}

void (*boot_DynaLoader_dyn)(pTHX, CV* cv);
CV* (*Perl_newXS_dyn)(pTHX, const char*, XSUBADDR_t, const char*);

void xs_init(pTHX) {
    static const char file[] = __FILE__;
    // dXSUB_SYS;            /* dNOOP */
    // PERL_UNUSED_CONTEXT;

    /* boot_DynaLoader */
    boot_DynaLoader_dyn = (void (*)(pTHX, CV* cv)) dlsym(handle, "boot
+_DynaLoader");
    if ( !boot_DynaLoader_dyn ) {
      fprintf( stderr, "(boot_DynaLoader) %s\n", dlerror() );
      exit(EXIT_FAILURE);
    }

    /* newXS is just Perl_newXS(aTHX, ...) */
    Perl_newXS_dyn = (CV* (*)(pTHX, const char*, XSUBADDR_t, const cha
+r*)) dlsym( handle, "Perl_newXS" );
 
    /* The following comment is mandatory. */
    /* DynaLoader is a special case */
    (*Perl_newXS_dyn)( aTHX, "DynaLoader::boot_DynaLoader", *boot_Dyna
+Loader_dyn, file );
}
[download]

Thanks for your help!