You need to convert the strings to which $pback, $set and "*/$testqueue[0]" evaluate into shell literals. Or simply avoid the shell entirely.

open(my $pipe, '-|', (
    $pback => ( $set, "*/$testqueue[0]" )
)
    or die("Can't launch $pback: $!\n");

my $out = '';
{ local $/; $out = <$pipe>; }
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