Your variables $1, $2, etc are read-only variables. In the foreach loop the $i is really a direct reference to $1 .. $5. So by modifying $i you are really trying to modify $1 .. $5, and you cannot do that. Here is an example:

$_ = "##";
m/(##)/;
$1 = "Hello";
Modification of a read-only value attempted at - line 3.
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To Fix your problem copy the read-only values to writeable variables:

@list = ($1, $2, $3, $4, $5);
foreach $i (@list)
{
    $i = int rand( @numb +1 );
     pop @numb;
}
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