Hidden features of Perl?

Answer 1

The "desperation mode" of Perl's loop control constructs which causes them to look up the stack to find a matching label allows some curious behaviors which Test::More takes advantage of, for better or worse.

SKIP: {
    skip() if $something;

    print "Never printed";
}

sub skip {
    no warnings "exiting";
    last SKIP;
}

There's the little known .pmc file. "use Foo" will look for Foo.pmc in @INC before Foo.pm. This was intended to allow compiled bytecode to be loaded first, but Module::Compile takes advantage of this to cache source filtered modules for faster load times and easier debugging.

The ability to turn warnings into errors.

local $SIG{__WARN__} = sub { die @_ };
$num = "two";
$sum = 1 + $num;
print "Never reached";

That's what I can think of off the top of my head that hasn't been mentioned.