my $debug = 1;
sub get_elevation_from_coordinates {
    my ($lat, $lon, $debug) = @_;

    $debug //= 0;
    ...
}

##</code><code>##

our $debug = 1; # OUR!
sub get_elevation_from_coordinates {
    my ($lat, $lon, $debug) = @_;

    $debug //= $main::debug; # if none specified, use "global" (main's)
    ...
}