How about this?

$line =~ s/39/3$fields[0]/;
$line =~ s/39/3$fields[1]/;
$line =~ s/39/3$fields[2]/;
$line =~ s/39/3$fields[3]/;
[download]

Update: This answer is getting voted down a lot. Maybe I am blind today, but could someone point out what is wrong with it?