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};