Non-bracketing delimiters use the same character fore and aft, but the four sorts of brackets (round, angle, square, curly) will all nest, which means that q{foo{bar}baz} is the same as 'foo{bar}baz'

The code falls into this category because the parens are balanced. If you unbalanced the parentheses(sp?) then you would end up with a syntax error.

$string = qq(           # start the quote
             func(1     # push down to second level
                   ln(  # push down to third level
                      e) # pop up to second level
                  );     # pop up to first level
            );           # finish the quote
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$a = $b + $c * ($d + $e * ($f + $g * ($h +)));
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Revealing tests cases include (with their effects):

1) try with too many right brackets (perl will generate a syntax error on an attempt to nest to a depth of -1).

2) try with too many left brackets (perl will complain using words like 'can't find a string terminator ")"'

3) use a different nesting or non-nesting delimiter from the content and mismatch the brackets in the content as before, e.g. qq{(((((()} (perl will have no problem with this despite the mismatching brackets, because they are of a different type than the delimiter being declared for qq.)

If you run this:

use strict;
use warnings;

my $test = qq(a (b c d);

print $test;
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You get this error:

Can't find string terminator ")" anywhere before EOF at math1.pl line 
+4.
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Which clearly indicates that perl follows the depth (matching) of '()' pairs.