You should look in perldoc perlop in the "quotes and quote-like operators" section... but the short explanation is:

• double quotes "..." will expand variables.
• single quotes '...' will not expand variables.
• backticks `...` will expand variables AND execute the string as though it were a command typed at the shell, AND THEN take the output of that command as the value of `...`.

For example: $x = "oo"; "f$x" (or just "foo") would yield the same result as 'foo', which would also yield the same result as `echo foo` (well... except that echo would probably put a newline on the end, so it would be like "foo\n"... but whatever).