Here's some explanatations and links (the useful kind, I hope):

• unless is a negated if. Just like if executes certain code if a condition is true, unless executes it if it is not - that is to say, it executes the code unless the condition is true. See Compound Statements for more.

• =~ is the regular expression binding operator. With a "regular" regular expression (rather than a substitution or transliteration), it matches a variable ($basename) against a regular expression. If you're not familiar with regular expressions, think of it as a pattern describing what the contents of $basename should look like. See perlop.html#Binding-Operators for more on the =~ operator, and perlretut and perlre for more on regular expressions.

If you're new to Perl, there's a number of helpful books as well, e.g. Learning Perl and Beginning Perl; I also highly recommend Programming Perl, affectionally known as "The Camel". (There's many more books, but they're not necessarily well-suited for beginners.)