perl -e "print qq/Hello world!\n/;"
Notice I used the qq// quotes instead of "" (double quotes). This was done because the entire string of code is wrapped in double-quotes, and it would confuse the command shell if you start embedding other similar quotes within the one liner. Some OS's allow for escaping, but it's easier just to avoid that issue altogether by using alternate quotes.
And, as I recently learned, any character can be used as the quotes if you leave a whitespace after the qq. My favorite now is to use qq qq... which makes for nice obfu (especially if you are quoting NUL and type it just as I did here). Just don't use q if q is part of the quoted text. Likewise, don't use / if / is part of the quoted text... (remembering to escape can be a real pain)
Linux, sci-fi, and Nat Torkington, all at Penguicon 3.0
perl -e 'print(map(chr,(0x4a,0x41,0x50,0x48,0xa)))'