Warnings go to STDERR, a file handle that is already open.
You can redirect them somewhere, as in below example to a file:

close(STDERR)
 or die qq(This should not go wrong, but this happened: $!\n);
my $warnings = qq(pid.$$.err_warn); # file to contain warnings & error
+ messages
open(STDERR, ">", $warnings)
 or die qq(unable to open "$warnings" for writing because of: $!\n);
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Cheers, Sören