Or, from the man page:

It has two separate effects. First, it automatically chomps $/ (the input record separator) when used with -n or -p. Second, it assigns "$\" (the output record separator) to have the value of octnum so that any print statements will have that separator added back on. If octnum is omitted, sets "$\" to the current value of $/.

Or, in the vernacular, if you include the -l option, it will add an end-of-line to each print.