It is perl telling you that the program used a uninitialized value in a comparison:

$ perl
use strict;
use warnings;

my $i;   # uninitialized
my $j=5;

if ($i == $j) {
  print "equal";
}
__END__
Use of uninitialized value in numeric eq (==) at - line 7.
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Examine the source code for DBIx::Class::Storage::DBI and see if you can spot the line in question.

You will only see such warnings when use warnings are specified. Do that. Always.