su - softwareUser -c /path/to/my/script.pl
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This should work, but you may also be able to use: fork, POSIX::setuid, exec(). I've used this on with perl 5.8.5 on AIX.

Opening to a file with a variable should work fine. You have this...

my $confgiFile = '/path/to/configFile';

open ( CONFIG, ">$config" ) or die "FATAL :: failure configuring - $!\
+n\n";

print CONFIG $config; # $config is populated much earlier

close(CONFIG);
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So you are setting $confgiFile to a path, but you are opening $config. Are these two the same here (e.g. is this a typo)? One thing I'd suggest is to put the file name that you pass to 'open' in the 'die' as well so you can sanity check what it is trying to do. The only other thing I can suggest is to check your umask just before the open, though I suspect it's probably ok.