In Perl regex, \b normaly matches a word boundary, but withing a character class it matches a backspace.
Generic whitespace \s
Is a shorthand for a whole character class that matches any whitespace character. This includes space, tab, newline, and carriage return.
The add i in m/../i instructs Perl to do the match in a case-insensitive manner. It's not acctualy part of the regex, but part of the m/../ syntatic packaging.
Among the more useful shorthands that Perl provide are:
\t a tab character
\n a newline character
\r a carriage-return character
\s any whitespace character (space, tab, newline)
\S anything not s
\w [a-zA-Z0-9_]+ match a word with w+
\W [^a-zA-Z0-9_]+ anything not w
\d [0-9] a digit
\D [^0-9] anything not d