When I run this code with warnings, it doesn't do what I expect, but I do get warnings:

use warnings;
use strict;
use 5.10.0;

my $ext = 'jpg';

given ($ext) {
    when (qw/jpeg jpg png/) {
        print "ok\n";
    }
    default {
        print "not ok\n";
    }
}
[download]

Useless use of a constant (jpeg) in void context at 
Useless use of a constant (jpg) in void context at 
not ok
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So I used B::Deparse:

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";
    }
}
[download]

That looks strange. So, I read Switch statements, and I saw an example of an array ref in a "when", and this seems to work:

    when ([qw/jpeg jpg png/]) {
[download]

Perhaps a more knowledgeable monk can explain why.