perl
-i    # edit in place
.bak  # rename original file by appending .bak to filename; print to new file with original name 
-p    # use a while loop to read each file; print each line (is stored in $_) which you have read (and modified)
e     # excecute code from command line
'     # start of code to be executed (for the default while loop)
$_    # default variable, each line of file
.=    # append the following to yourself ($_) and assign it to you 
"\n"  # string to be appended
'     # end of code