in reply to STDERR going to string

Ok, this is the error that you want to catch, two "," at the end of the line. This is not a perl error, is a bibtex error of course, perl should know nothing about this (but Text::BibTeX should).

TITLE  = {Arranging Arguments},,

and if I comment all the fuss about STDERR and simply modify this line I got the bibtex error correctly

my $entry = new Text::BibTeX::Entry $bib_entry or die $!;

Update: Is obvious after reading the other answers that I had not understood the question at all...