It's generated by code something like these:

$ perl -wE 'my $vals; my $y = index($vals, "x")'
Use of uninitialized value $vals in index at -e line 1.
$ perl -wE 'my $vals; my $y = index("x", $vals)'
Use of uninitialized value $vals in index at -e line 1.
[download]

Look in /homedir/perl5/lib/perl5/CGI.pm, go to line 1243, and start investigating from there.