Idiomatically,

my $btn = param ('btn');
if (defined ($btn) && $btn eq 'btn') {
    ...
}
[download]

or

use 5.010;
my $btn = param ('btn') // '';
if ($btn eq 'btn') {
    ...
}
[download]

Or see warnings for taking off the seatbelts - but don't even think about doing that until you understand what you're doing.