You need to drop the qw() and change $parameters to an array e.g

my @parameters = (colorspace = >'gray', compression = >'none');
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Because as it stood $parameters was being assigned compression=>'none' as your code really looked like this

my $parameters = (q[colorspace=>'gray'], q[compression=>'none']);
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See. perlop for more info on the workings of qw().
HTH

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broquaint