You should first check out the perl documentation, where you'll find links to the answers to your questions (start with the Overview and Tutorials sections), e.g. $ does preceed scalar variables (see perldata), and variables are defined as you use them (although you are strongly encouraged to Use strict and warnings, "/gio" is actually part of "=~" with "//" (with the g,i, and o options) which is the pattern match operator (see perlop and perlre), All of these docs can be displayed at the command line with perldoc, e.g. "perldoc perl" or "perldoc perlre".

There are also numerous Tutorials on this site.