Assuming you know the line number after which you want to add the new line (say 2; in your example it's 1), you could do something like:

my $linenum=2;
my $linetoadd="Something\n";
my $i;
while(<>){
 ++$i;
 if($i==$linenum) {print;print $linetoadd;}
 else {print;}
 }
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(You can direct the output to a new file.)
You could also arrange things so the line number and new line are given as args, depending on how this is to be used. (i.e a one time run or or a utility used frequently; of course, if it's just a one time thing, I'd simply use an editor.)
Hope this is what you wanted.
chas