use Data::Dumper;
$Data::Dumper::Deparse = 1;

my $sub;
{
    my $x = "foo";
    $sub = sub {
        my $filename = shift;
        return "/tmp/$x/" . $filename;
    };
}

print Dumper($sub);
__END__
$VAR1 = sub {
            my $filename = shift @_;
            return "/tmp/$x/" . $filename;
        };