$ perl -Mstrict -Mwarnings -E'
my $foo;
say (defined $foo ? "1: defined" : "1: not defined");
my $bar = $foo;
say "2: bar: $bar";
my $baz = $foo || "baz";
say "3: baz: $baz";
my $qux = $foo // "qux";
say "4: qux: $qux";
'
1: not defined
Use of uninitialized value $bar in concatenation (.) or string at -e line 5.
2: bar: 
3: baz: baz
4: qux: qux