How can I count the number of occurrences of a substring within a string?

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";