Please let me to improve your code: 
my $s = 'xx @ra[1] y @ara[23] z @ra[1] aa @ara[23] b @ra[1] @ara[23]';
print qq{'$s'\n};

my %found;
my %generated;
$s =~ s{ \@ ([a-z] [a-z\d_]* \[) \K (\d+) (?= \]) }
       {
            $found{"$1$2"} //=
        do {
            my $rand;
            do {$rand = int rand 9999} while exists $generated{$rand};
            $generated{$rand} = ();
            $rand;
            };
       }xmsge;

print qq{'$s'\n};

[download]
Fixed issues of producing something like this: 
'xx @ra[1] y @ara[23] z @ra[1] aa @ara[23] b @ra[1] @ara[23]'
'xx @ra[1] y @ara[0] z @ra[1] aa @ara[2] b @ra[1] @ara[2]'

[download]

In reply to Re^2: Same problem replacing array index numbers by Anonymous Monk
in thread Same problem replacing array index numbers by astroid96

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