s/^\s*;\s*//$_ against this pattern:
the beginning of the string (^) followed by zero or
more whitespace characters (\s*), then a semicolon
(;), then possibly some more whitespace (\s*).
If it matches, the matching part of the string is stripped out (ie,
substituted with nothing) and the whole expression has a true value.
If not, it just has a false value.Recall:
s///) are true if the
substitute pattern applied and was performed and false if they
weren't.