$filein=yourcvsfile.txt;
$fileout=yourcvsfileout.txt;
####read into array
open (FILEIN,"$filein") || die "Can't Open  file : $!\n";
@mydata=<FILEIN>;
close FILEIN;
####open output file
open (FILEOUT,">$fileout") || die "1Can't Open b: $!\n";


foreach $mydata (@mydata) {($newfield1,$newfield2,$newfield3,$newfield4,$newfield5) = split(',',$newline);

if ($newfield1 eq $newvar)
 {$field1=$var1;
###preform sub routine to write
&writenew};

sub writenew {
$newfield1=$var2;
$newfield2=$var2
$newfield3=$var2
$newfield4=$var2
$newfield5="anydata";
$mydata="$newfield1,$newfield2,$newfield3,$newfield4,$newfield5; "
print FILEOUT $mydata;

}