Is there a Perl shortcut to count the number of matches in a string?

I think the clearest way to describe this would be to avoid the instant-cast to scalar. First assign to an array, and then use that array in scalar context. That's basically what the = () = idiom will do, but without the (rarely used) idiom:

my $string = "one.two.three.four";
my @count = $string =~ /\./g;
print scalar @count;

Also, see Perlfaq4 :

There are a number of ways, with varying efficiency. If you want a count of a certain single character (X) within a string, you can use the tr/// function like so:

$string = "ThisXlineXhasXsomeXx'sXinXit";
$count = ($string =~ tr/X//);
print "There are $count X characters in the string";

This is fine if you are just looking for a single character. However, if you are trying to count multiple character substrings within a larger string, tr/// won't work. What you can do is wrap a while() loop around a global pattern match. For example, let's count negative integers:

$string = "-9 55 48 -2 23 -76 4 14 -44";
while ($string =~ /-\d+/g) { $count++ }
print "There are $count negative numbers in the string";

Another version uses a global match in list context, then assigns the result to a scalar, producing a count of the number of matches.

$count = () = $string =~ /-\d+/g;

Is the following code a one-liner?

print $string =~ s/\./\./g;

That puts the regex itself in scalar context, which isn't what you want. Instead, put the regex in list context (to get the number of matches) and put that into scalar context.

 my $number = () = $string =~ /\./gi;