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";
    }
}
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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";
    }
}
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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/]) {
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Perhaps a more knowledgeable monk can explain why.

Good job tracking this down. It sounds like another way that "smart" match is broken.

Because qw// constructs a list not an array.

When you did this:

given ($ext) {
    when (qw/jpeg jpg png/) {
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You should have seen the warning:

Useless use of a constant in void context at
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You could do it in-line as:

given ($ext) {
    when ( @{[ qw/jpeg jpg png/] ]} ) {
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But it's a bit cheesy.

What's the difference between a list and an array, as far as when is concerned? What's an example of a list used here that is different from the meaning of an array with the same elements?

What's the difference between a list and an array, as far as when is concerned?

This is one of the ways that given/when break expectations in that in most cases, the use of an array expression @a generates a list.

The only other exception to this I can think of is a subroutine with a prototype of (\@) which causes a reference to the array to be passed to the subroutine. I think that's what is happening here, though the documentation leave much to be desired.

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

Perl perlsyn

"Another useful shortcut is that, if you use a literal array or hash as the argument to given , it is turned into a reference. So given(@foo) is the same as given(\@foo) , for example."

Therefore, just use an array reference like already suggested by toolic:

 when ([qw(jpeg jpg png)]) {
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Or use a regex

 when (/^(?:jpeg|jpg|png)$/) {
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#!/usr/bin/perl

use feature qw/:5.10 switch say/;

my(@gr_choices) = qw/jpeg jpg png/;
given($gr_choices[0]) {
    when('jpeg') {
        say 'jpeg is one of the choices';
    }
}
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