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

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;
use feature 'say';
use feature 'signatures';
no warnings 'experimental::signatures';

use Data::Dump qw( pp );

sub foo( $param ) {
    say "\$param is ", pp( $param );

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

    say "\$my_var after declaration is ", pp( $my_var );
    say +( defined $param
            ? ! defined $my_var
            : defined $my_var && $my_var eq 'default' )
        ? "    (as expected)"
        : "    (unexpected to me!)";

    $my_var //= $param;

    say "\$my_var after '//=' is ", pp( $my_var );
    say "";
    return $my_var;
}

foo( $_ )
    for undef, "call 1", "call 2", "call 3";

1;

##</code><code>##

$ ./my_with_if.pl
$param is undef
$my_var after declaration is "default"
    (as expected)
$my_var after '//=' is "default"

$param is "call 1"
$my_var after declaration is undef
    (as expected)
$my_var after '//=' is "call 1"

$param is "call 2"
$my_var after declaration is "call 1"
    (unexpected to me!)
$my_var after '//=' is "call 1"

$param is "call 3"
$my_var after declaration is "call 1"
    (unexpected to me!)
$my_var after '//=' is "call 1"