I didn't write the data_sources method but I look after DBD::ODBC these days. I don't see a 64bit issue and inspecting the code I cannot see any issue although there are things I might have done differently.

I still suspect it is something to do with the large number of DSNs you have. If you download DBD::ODBC and build it yourself I can suggest a few small changes that might help identify the issue.

Download the latest DBD::ODBC, and untar it into a dir. Open a strawberry perl shell session and change to the dir above. Edit the file ODBC.xs and delete the function data_sources and replace it with the one below:

void
data_sources(drh, attr = NULL)
    SV* drh;
    SV* attr;
  PROTOTYPE: $;$
  PPCODE:
    {
#ifdef DBD_ODBC_NO_DATASOURCES
        /*    D_imp_drh(drh);
            imp_drh->henv = SQL_NULL_HENV;
            dbd_error(drh, (RETCODE) SQL_ERROR, "data_sources: SOLID d
+oesn't implement SQLDataSources()");*/
        XSRETURN(0);
#else
    int numDataSources = 0;
    SQLUSMALLINT fDirection = SQL_FETCH_FIRST;
    RETCODE rc;
    SQLCHAR dsn[SQL_MAX_DSN_LENGTH+1+9 /* strlen("DBI:ODBC:") */];
    SQLSMALLINT dsn_length;
    SQLCHAR description[256];
    SQLSMALLINT description_length;
    D_imp_drh(drh);

    if (!imp_drh->connects) {
        rc = SQLAllocEnv(&imp_drh->henv);
        if (!SQL_ok(rc)) {
            imp_drh->henv = SQL_NULL_HENV;
            dbd_error(drh, rc, "data_sources/SQLAllocEnv");
            XSRETURN(0);
        }
    }
    strcpy(dsn, "dbi:ODBC:");
    while (1) {
        description[0] = '\0';
        rc = SQLDataSources(imp_drh->henv, fDirection,
                            dsn+9, /* strlen("dbi:ODBC:") */
                            SQL_MAX_DSN_LENGTH,
                            &dsn_length,
                            description, sizeof(description),
                            &description_length);
        if (!SQL_ok(rc)) {
            if (rc != SQL_NO_DATA_FOUND) {
                /*
                 *  Temporarily increment imp_drh->connects, so
                 *  that dbd_error uses our henv.
                 */
                imp_drh->connects++;
                dbd_error(drh, rc, "data_sources/SQLDataSources");
                imp_drh->connects--;
            }
            break;
        }
        printf("/%s/ (%d), /%s/ (%d)\n",
               (dsn + 9), dsn_length,
               description, description_length);
        ST(numDataSources++) = newSVpv(dsn, dsn_length+9 /* strlen("db
+i:ODBC:") */ );
        fDirection = SQL_FETCH_NEXT;
    }
    if (!imp_drh->connects) {
        SQLFreeEnv(imp_drh->henv);
        imp_drh->henv = SQL_NULL_HENV;
    }
    XSRETURN(numDataSources);                   /* return indicating #
+ items on stack */
#endif /* no data sources */
    }
[download]

Now type perl Makefile.PL, then dmake and it should build you a DBD::ODBC. Now run something like the following:

perl -Ic:\path_to_dbd_odbc_dir\blib\lib -Ic:\path_to_dbd_odbc_dir\blib
+\arch -le "use DBI; my @a = DBI->data_sources('ODBC');"
[download]

Replace c:\path_to_dbd_odbc_dir as per your machine and where you unpacked DBD::ODBC. It should output a list of your dsns and lengths - what was the output and did it crash? If it does not crash, take the printf line out (all from printf to the semi colon), rebuild and try again? Does it work now?

Thank you a lot.

I am not used to compiling packages, since i am used to use ActiveState Perl for Windows and am used to get ready to use ppm-packages, so please forgive me.

I did as described, but on doing the "perl Makefile.PL" it is telling me:

Press return to continue...  
Using DBI 1.631 (for perl 5.018001 on MSWin32-x64-multi-thread) instal
+led in C:/strawberry/perl/site/lib/auto/DBI/
Strawberry Perl found; adjusting the INC variable;
Using DBI 1.631 (for perl 5.018001 on MSWin32-x64-multi-thread) instal
+led in C:/strawberry/perl/site/lib/auto/DBI/
INC is now INC =  -IC:\strawberry\perl\site\lib\auto\DBI -IC:\strawber
+ry\perl\site\lib\auto\DBI
Using DBI 1.631 (for perl 5.018001 on MSWin32-x64-multi-thread) instal
+led in C:/strawberry/perl/site/lib/auto/DBI/
Writing Makefile for DBD::ODBC
Writing MYMETA.yml and MYMETA.json
Warning: not all required environment variables are set.

Warning: Will not be able to run tests as you have not defined
all of DBI_DSN, DBI_USER and DBI_PASS environment variables.
[download]

What values should i set here (and what are they for)? It also gave me Warning (mostly harmless): No library found for ODBC32.LIB.

Ignore the first warning - you don't need to run any tests so you don't need to set those env vars. I'm not sure about the second one but just do dmake and see where that gets you.