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use List::Util qw( shuffle );

...
    return if $start > $end or $num > $end - $start + 1;
    ( shuffle $start .. $end )[ 0 .. $num - 1 ];
}

use strict;
use warnings;
...
        "retry($n,$s,$e)"   => sub { randseq_retry   $n, $s, $e },
    };
}