Trying to join twice is fatal.

C:\>perl -Mthreads -e "my $t = threads->new( sub {} ); $t->join; $t->j
+oin;die 2"
Thread already joined at -e line 1.
[download]

You can trap fatal errors with eval. perldoc -f eval

C:\>perl -Mthreads -e "my $t = threads->new( sub {} ); $t->join; eval{
+$t->join};warn $@;die 2"
Thread already joined at -e line 1.
2 at -e line 1.
[download]

 

thread exited while x threads were running

That is a warning (not-fatal), not an error, and you can largely ignore it.

perldoc perldiag
perldoc diagnostics

update:
Also, $Config{useithreads} or die("Recompile Perl with threads to run this program."); is useless as use threads; will fail if useithreads isn't defined.