>perl -wMstrict -le
"my $s =
   'xx @ra[1] y @ara[23] z @ra[1] aa @ara[23] b @ra[1] @ara[23]';
 print qq{'$s'};
 ;;
 my %found;
 $s =~ s{ \@ ([a-z] [a-z\d_]* \[) \K (\d+) (?= \]) }
        { $found{$1 . $2} //= int rand 9999;       }xmsge;
 print qq{'$s'};
"
'xx @ra[1] y @ara[23] z @ra[1] aa @ara[23] b @ra[1] @ara[23]'
'xx @ra[2038] y @ara[6310] z @ra[2038] aa @ara[6310] b @ra[2038] @ara[6310]'