I think that the reference trick for CGI.pm's html generators would qualify. For example,

use CGI ':standard';
my @foo = qw/foo bar baz/;

print p({}, @foo), $/;
print p({}, \@foo), $/;

__END__
<p>foo bar baz</p>
<p>foo</p> <p>bar</p> <p>baz</p>
[download]

That's pretty puzzling to somebody who doesn't know that CGI.pm convention. Very useful if you have to build a table on the fly.

The need for that has pretty much disappeared with the adoption of powerful Template mechanisms.