This is basically a s/ubsti/tute/ operation (see Regexp Quote Like Operators operator, regexp in perlop) with the modifiers ige but in alternative syntax: the /patten/replacement/ elements get delimited with braces s{}{}, then the substitution is broken up with whitespace for better readability. The susbtitution part is actually a piece of code executed when the pattern is found, and which result gets interpolated for the pattern. That's what the e modifier stands for.