Q: What is the effect when I drink a pint of beer?
A: I get slightly tipsy.
But that's too simplistic!
A: The brewery has some more money
A: There's a glass to wash up
A: I need the loo!
Running a piece of code is like drinking a pint of beer - as well as a headline result, you get extra variables / side effects ... and the normal assignment statement (the = sign in all the languages we teach except Tcl) has a single target.
In Perl, though, side effect variables can be set too - and that's done especially with a regular expression match. The 'headline' return value is a true or false return, telling whether or not the string matched a patter, but there's more available to you if you want to use it:
$1, $2 etc ... for 'interesting bits' that matched parts of the pattern
$`, $& and $' for the bits before and after the matched part and the match itself
And if you use a match in a list context, you'll get back a whole list of all the interesting bits ... matched many times over if you have used the "g" modifier on the match.
From today's course ... source code that shows lots of these extra variables (written 2009-06-11, updated 2009-06-12)
Associated topics are indexed underP212 - Perl - More on Character Strings 
First match or all matches? Perl Regular Expressions - (2012-11-19) 
Converting codons via Amino Acids to Proteins in Perl - (2012-04-25) 
Possessive Regular Expression Matching - Perl, Objective C and some other languages - (2012-03-12) 
Serialsing and unserialising data for storage and transfer in Perl - (2012-02-28) 
The difference between dot (a.k.a. full stop, period) and comma in Perl - (2011-12-09) 
Single and double quotes strings in Perl - what is the difference? - (2011-08-30) 
DNA to Amino Acid - a sample Perl script - (2011-06-24) 
How much has Perl (and other languages) changed? - (2011-06-10) 
Looking ahead and behind in Regular Expressions - double matching - (2010-12-23) 
Object Orientation in an hour and other Perl Lectures - (2010-11-18) 
Arrays v Lists - what is the difference, why use one or the other - (2010-10-10) 
Further more advanced Perl examples - (2010-07-19) 
Unpacking a Perl string into a list - (2010-07-16) 
Teaching examples in Perl - third and final part - (2010-06-27) 
Binary data handling with unpack in Perl - (2010-06-10) 
Want to do a big batch edit? Nothing beats Perl! - (2010-03-01) 
Making variables persistant, pretending a database is a variable and other Perl tricks - (2009-08-27) 
Perl substitute - the e modifier - (2008-12-16) 
Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03) 
Equality and looks like tests - Perl - (2008-07-29) 
Handling Binary data (.gif file example) in Perl - (2008-01-17) 
Ignore case in Regular Expression - (2007-09-08) 
Regular expressions made easy - building from components - (2007-08-16) 
Substitute operator / modifiers in Perl - (2007-06-28) 
Commenting a Perl Regular Expression - (2007-06-12) 
Perl, the substitute operator s - (2007-06-08) 
Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25) 
C++ and Perl - why did they do it THAT way? - (2006-11-16) 
Coloured text in a terminal from Perl - (2006-05-29) 
Don't expose your regular expressions - (2006-02-15) 
Storing a regular expression in a perl variable - (2006-02-09) 
Perl Regular Expressions - finding the position and length of the match - (2006-02-02) 
Remember to process blank lines - (2006-01-31) 
Commenting Perl regular expressions - (2005-09-30)
Some other Articles
Loading external code into Perl from a nonstandard directoryTransforming data in Perl using lists of lists and hashes of hashesWhy sendmail one way, and pop3 the other?What is CGI.pm / A dozen new examplesRunning a piece of code is like drinking a pint of beerDo not re-invent the wheel - use a Perl moduleWhere do I start when writing a program?Learning PHP, Ruby, Lua and Python - upcoming coursesRevision / Summary of lists - PerlHow important is a front page ranking on a search engine?