xdev$ cat foo
line 1
line 2
line 3
xdev$ cat foo1
line 1
line 2
line 3
xdev$ perl -i.bak -ne 'if ($.==1){print "\n";}print;if (eof){close ARGV;}' foo* 
xdev$ cat foo

line 1
line 2
line 3
xdev$ cat foo1

line 1
line 2
line 3
xdev$