I agree with others here that you should first and foremost get your hands dirty and just play with regexes. Just use a lot of capturing and print out what you just captured like so:

my $string = "testing, testing, 123";
my @captured = ($string=~/(testing)/g);
print "captured: $_\n" for @captured;
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All you need to know here is that the parentheses capture the string that was matched and the array will contain all matched strings from this regex. The /g means "don't stop after first match, keep searching the entire string". Then play with the regex a bit. Be sure to try character groups such as [a-z], try greedy vs non-greedy:

# match lowercase letters commas and space and get as many as possible
+ (greedy)
$string=~/([a-z,\s]+)/; 

# same but non-greedy (stop as soon as you can)
$string=~/([a-z,\s]+?)/;
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try anchors like ^ (beginning of string) and $ (end of string). That will get you started.