use warnings;
use strict;

use Data::Dumper;

open my $log_fh, '>', 'log.txt' or die $!;

my @HR = (1, 2, undef, 3);
my $thing = undef;
my $blah = 99;

{
    local $SIG{__WARN__} = sub {
        if ($_[0] =~ /uninitialized/){
            my $warn = {
                '@HR' => \@HR,
                '$thing' => $thing,
                '$blah' => $blah,
            };
            print $log_fh Dumper $warn;
            print $log_fh "\n";
        }
    };

    my $x = sprintf $thing, @HR;
}