from http://perldoc.perl.org/perlre.html:

By default, a quantified subpattern is "greedy", that is, it will match as many times as possible (given a particular starting location) while still allowing the rest of the pattern to match. If you want it to match the minimum number of times possible, follow the quantifier with a "?" . Note that the meanings don't change, just the "greediness":

$1 leaves space so that the other terms can match.

Update, consider:

my $x = "This is Perl";
$x =~/^((.+)(e|r)(.*))$/;
print "1={$1}  2={$2}  3={$3}  4={$4}\n";
# 1={This is Perl}  2={This is Pe}  3={r}  4={l}

my $x = "This is Perl, nice Perl";
$x =~/^((.+)(e|r)(.*))$/;
print "1={$1}  2={$2}  3={$3}  4={$4}\n";
# 1={This is Perl, nice Perl}  2={This is Perl, nice Pe}  3={r}  4={l}
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A small update, I changed $a to $x in the above code. In Perl, $a and $b are special variables used for among other things in sort functions. Normal user code should not use these variables except in their strange special cases. So something like $x and $y is better. In the above code using $a wouldn't matter, but I changed it anyway to point out that this is a bad habit that can lead to problems in longer programs. Just something to watch out for if you code in other languages that don't have special meanings for a or b.