$ perl -Mstrict -Mwarnings -e '
    use 5.018;
    use feature "lexical_subs";
    no if $] >= 5.018, warnings => "experimental::smartmatch";
    no if $] >= 5.018, warnings => "experimental::lexical_subs";
    my ($x, $y) = (1, undef);
    given ($x) {
        when (1) { say "\$x == 1" }
        default  { say "\$x != 1" }
    }
    given ($y) {
        when (1) { say "\$y == 1" }
        default  { say "\$y != 1" }
    }
    my sub z { say "In z()" }
    z();
'
$x == 1
$y != 1
In z()