Just to expand on previous answer with some example. An operator of the form $LHS <OPERATOR>= $RHS (I think the general name is "shorthand"?) is the shorthand for $LHS = $LHS <OPERATOR> $RHS. (Edit: s/LHW/LHS/) E.g., the long way is: my $x = undef; my $x = $x // 12;. The shorthand is $x //= 12;. Many other operators follow this style, for example $x = 12; $x += 3; # $x is 15. But also $x = 0; $x ||= 1; Also, $x = 10; $x /= 2; # 5

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

    $debug //= 0;
    ...
}
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Now, in the context of the above quoted code which I think you refer to. There are 2 $debug there! One belongs to the outer scope of the sub. It will be valid inside the sub unless the sub declares (my $debug) its own private one. In this case it's an optional sub parameter who is set to zero if left undef (by the caller of the sub, e.g. in this case: get_elevation_from_coordinates(1,2)). That code would have been better like this:

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

    $debug //= $main::debug; # if none specified, use "global" (main's
+)
    ...
}
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In which case there's a global debug which is used to set its sub's debug value only if the caller did not specify one.

This document is helpful to me: https://perl.plover.com/FAQs/Namespaces.html

bw, bliako