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    my $my_var = "default"
        unless defined $param;
    $my_var //= $param;

#!/usr/bin/env perl

...
    for undef, "call 1", "call 2", "call 3";

1;

$ ./my_with_if.pl
$param is undef
...
    (unexpected to me!)
$my_var after '//=' is "call 1"