( $chapter =~ /5/ )  and ( $chapter = 5 ); The second part of the "and" seems to be assigning "5" to $chapter but I'm normally used to the first part as being "If $chapter matches 5 then do something".

Yes, your interpretation is correct. Perl's low-precedence boolean operators and/or/etc. can be used in place of if conditionals; the statement you showed is the equivalent of if ( $chapter =~ /5/ ) { $chapter = 5 } (and could arguably be written this way for better readability*). =~ is the binding operator in this case saying to match the regex on its right to the string on its left, and = is a simple assignment.

* Yet another way to write that is using Statement Modifiers: $chapter = 5 if $chapter =~ /5/;, if you find that more readable. In fact, this is what Perl optimizes the above statement to:

$ perl -MO=Deparse -e '( $chapter =~ /5/ ) and ( $chapter = 5 );'
$chapter = 5 if $chapter =~ /5/;
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Note that I'd say the only difference between the statements being discussed here is readability, so since you had to ask about the syntax, you may prefer to rewrite the statement in one of the above forms if that helps you to be able to read the code more easily.