Many thanks for this. We may be getting there. I'm not too sure how to make the change with regard the IIS user. However, I've done the CGI::Carp thing and a page came back thus:

Software error:

install_driver(mysql) failed: Can't load 'C:/strawberry/perl/vendor/li
+b/auto/DBD/mysql/mysql.dll' for module DBD::mysql: load_file:The spec
+ified module could not be found at C:/strawberry/perl/lib/DynaLoader.
+pm line 200.
 at (eval 42) line 3
Compilation failed in require at (eval 42) line 3.
Perhaps a required shared library or dll isn't installed where expecte
+d
 at c:\inetpub\wwwroot\index_test.pl line 223
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For help, please send mail to this site's webmaster, giving this error message and the time and date of the error.

----------- END OF ERROR ---------------

I checked out line 200 of DynaLoader.pm and it contains this:

my $libref = dl_load_file($file, $module->dl_load_flags) or croak("Can
+'t load '$file' for module $module: ".dl_error());
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But then immediately before this is the following comment:

# Many dynamic extension loading problems will appear to come from
# this section of code: XYZ failed at line 123 of DynaLoader.pm.
# Often these errors are actually occurring in the initialisation
# C code of the extension XS file. Perl reports the error as being
# in this perl code simply because this was the last perl code
# it executed.
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Down the rabbit hole we go . . .

This most likely means that either DBD::mysql is not installed on your machine or that at least one of the DLLs required by DBD::mysql is not installed. You will likely need to install (and potentially set up) the MySQL client.

Do you have DBD::mysql installed on this server? Check out the instructions INSTALL file.

Update: fixed typo in module name, added link to INSTALL file.

For the record: yes, dbd::mysql was installed:

Please type in a space-separated list of modules you want to find
the installed versions for below.
> dbd::mysql

The version of dbd::mysql in C:/strawberry/perl/vendor/lib is 4.014

Press any key to exit.
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I think the problem was this weird .dll file referred to above. Many thanks for all your help.