Your request would be solved formally by

for Unix:
 perl -i.bak -0777 -lpe 's/\nef\n/\nef\na test content\n<blank line>\n
+/' a.txt

for Windows:
 perl -i.bak -0777 -lpe "s/\nef\n/\nef\na test content\n<blank line>\n
+/" a.txt
[download]

Addendum: If you only want to insert sth. after a specified line, then you may get along by doing this:

Unix:
 perl -i.bak -pe 'print "a test content\n<blank line>\n" if $.==4' a.t
+xt

Windows:
 perl -i.bak -pe "print qq{a test content\n<blank line>\n} if $.==4" a
+.txt
[download]

So here we count on $. (line number) and the line after #3 is #4.

Maybe you meant something else by your term "fseek in perl". Better you'd give a small hint what exactly you are trying to solve by that?

Regards

mwa