perl -MO=Deparse given.pl

Useless use of a constant (jpeg) in void context at 
Useless use of a constant (jpg) in void context at 
given.pl syntax OK
sub BEGIN {
    use warnings;
    use strict 'refs';
    require 5.10.0;
}
use warnings;
use strict 'refs';
BEGIN {
    $^H{'feature_say'} = q(1);
    $^H{'feature_state'} = q(1);
    $^H{'feature_switch'} = q(1);
}
my $ext = 'jpg';
given ($ext) {
    when ('???', '???', 'png') {
        print "ok\n";
    }
    default {
        print "not ok\n";
    }
}