The Binding Operator, =~ (Learning Perl, 3rd Edition)

9.3. The Binding Operator, =~

Matching against $_ is merely the default; the binding operator (=~) tells Perl to match the pattern on the right against the string on the left, instead of matching against $_.[195] For example:

[195]The binding operator is also used with some other operations besides the pattern match, as we'll see later.

my $some_other = "I dream of betty rubble.";
if ($some_other =~ /\brub/) {
  print "Aye, there's the rub.\n";
}

The first time you see it, the binding operator looks like some kind of assignment operator. But it's no such thing! It is simply saying, "this pattern match which would attach to $_ by default -- make it work with this string on the left instead." If there's no binding operator, the expression is using $_ by default.

In the (somewhat unusual) example below, $likes_perl is set to a Boolean value according to what the user typed at the prompt. This is a little on the quick-and-dirty side, because the line of input itself is discarded. This code reads the line of input, tests that string against the pattern, then discards the line of input.[196] It doesn't use or change $_ at all.

[196]Remember, the line of input is not automatically stored into $_unless the line-input operator (<STDIN>) is all alone in the conditional expression of a while loop.

print "Do you like Perl? ";
my $likes_perl = (<STDIN> =~ /\byes\b/i);
...  # Time passes...
if ($likes_perl) {
  print "You said earlier that you like Perl, so...\n";
  ...
}

The parentheses around the pattern-test expression aren't required, so the following line does the same thing as the one above -- it stores the result of the test (and not the line of input) into the variable:

my $likes_perl = <STDIN> =~ /\byes\b/i;