The most used special variable is
which is a default scalar variable.
To understand it better let's look at the examples.
You're familiar with
It prints whatever you pass as arguments.
But what happens when you don't pass any arguments?
It takes data from the default
$_ = 'Hello'; say;
Of course you don't usually need this functionality, but it can very useful when using loops, for example:
say for (1 .. 10);
for loop sets a
$_ variable and
say prints it.
Many embedded Perl functions use default variable when no arguments are passed.