When debugging warnings from the perl core like Use of uninitialized value ... let the debugger pause right there. Then have a good look at the context that led to this situation and investigate variables and the callstack.

To let the debugger do this automatically I use a debugger customization script:

sub afterinit 
{
    $::SIG{'__WARN__'} = sub {
        my $warning = shift;
        if ( $warning =~ m{\s at \s \S+ \s line \s \d+ \. $}xms ) {
            $DB::single = 1;    # debugger stops here automatically
        }
        warn $warning;
    };
    print "sigwarn handler installed!\n";
    return;
}
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Save the content to file .perldb (or perldb.ini on Windows) and place it in the current or in your HOME directory.

The subroutine will be called initially by the debugger and installs a signal handler for all warnings. If the format matches one from the perl core, execution in the debugger is paused by setting $DB::single = 1.