Without using regular expression,Following already gave me what I want.I just wanted to make it better using regular expression,but I couldn't think of any..:(

$ret=index($in{'cmd'},":");
if($ret eq -1) {
     @arr=split(/ /,$in{'cmd'});
     $delim=" ";
} else {
     @arr=split(/:/,$in{'cmd'}); 
     $delim=":";
}
$i=0;
while(@arr[$i]) {
    if(@arr[$i] == 1) {
         $cmd=$cmd.$in{'args1'}.$delim;
    }elsif(@arr[$i] == 2) {
         $cmd=$cmd.$in{'args2'}.$delim;
    }elsif(@arr[$i] == 3) {
         $cmd=$cmd.$in{'args3'}.$delim;
    }else {
         $cmd=$cmd.@arr[$i].$delim;
    }   
    $i++;   
}   
chop($cmd);
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12
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should become

args1args2
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