That has always bugged me. Personally I wish that Perl were a little less DWIM about:

$elem = @array[0];
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No, they act like lists in a scalar context, to be precisely equal to their definition in perldata.

-- Randal L. Schwartz, Perl hacker

I know what they do. I just think it would be better if they did something else. :-)

@exts{@exts} = @exts;
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Yes. See:

    my @exts = qw/foo bar baz/;
    my %exts;

    @exts{@exts} = @exts;

    use Data::Dumper;
    print Dumper \%exts;
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produces:

    $VAR1 = {
          'foo' => 'foo',
          'baz' => 'baz',
          'bar' => 'bar'
        };
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I might point out that nothing answers a question so well as testing a bit of code out yourself. :)