That's dereferencing of a glob reference, admittedly not a very common construct.

> ${*$ftp}{KEY}

That's accessing the %hsh for $ftp=\*hsh with $hsh{KEY}

> I've been programming in perl for quite some time now

you might have seen constructs like \*DATA before, those are glob references.

  DB<7> x \*DATA
0  GLOB(0x2d92040)
   -> *main::DATA
  DB<8> p ref \*DATA
GLOB
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I hope it's clearer now.

Bareword filehandles and globs are historically linked. (Perl4 legacy IIRC)

EDIT

or better phrased the slot for filehandles in typeglobs has no own sigil like other types, hence you can only use *glob for referencing them.

Thanks Rolf and all others! I did some digging and found this at the bottom of sub new in IO::Handle:

    my $io = gensym;
    bless $io, $class;
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The call to gensym is also new to me but I looked it up and now I understand what’s going on! —joe

No, $ftp->{...} would imply that $ftp is a hash reference. But it is not, it is a glob reference.

# code not tested
sub test_this {
  my $stuff = call_to_external_service();
  return $stuff;
}

#...

# subroutine mock for a unit test, e.g.,
{
  no warnings qw/redefine/;
  my $expected = { some => 'expected result'};
  local *call_to_external_service = sub { return $expected, };
  # now call a test or something that calls
  is_deeply test_this(), $expected, qw{whatever...};
}
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Monkey patching can be quite useful in another way, too. It allows to generate lots of functions in a loop instead of writing them one-by-one. I sometimes use this in some wrapper modules, so i can for example log all database calls for debugging.

BEGIN {
    # Auto-magically generate a number of similar functions without ac
+tually
    # writing them down one-by-one. This makes changes much easier, bu
+t
    # you need perl wizardry level +10 to understand how it works...
    my @simpleFuncs = qw(commit rollback errstr);
    my @stdFuncs = qw(do quote pg_savepoint pg_rollback_to pg_release)
+;

    for my $a (@simpleFuncs){
        no strict 'refs'; ## no critic (TestingAndDebugging::ProhibitN
+oStrict)
        *{__PACKAGE__ . "::$a"} = sub { $_[0]->checkDBH(); return $_[0
+]->{mdbh}->$a(); };
    }

    for my $a (@stdFuncs){
        no strict 'refs'; ## no critic (TestingAndDebugging::ProhibitN
+oStrict)
        *{__PACKAGE__ . "::$a"} = sub { $_[0]->checkDBH(); if($_[0]->{
+isDebugging}) {print STDERR $_[0]->{modname}, " ", $_[1], "\n";}; ret
+urn $_[0]->{mdbh}->$a($_[1]); };
    }
};
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perl -e 'use Crypt::Digest::SHA256 qw[sha256_hex]; print substr(sha256_hex("the Answer To Life, The Universe And Everything"), 6, 2), "\n";'

$a is a special variable. Don't use it for anything else.
map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

Just created a list of typeglob references: Re: Is typeglob feature really useful? (typeglob references)

The symbol table consists of a hierarchy of globs. A glob, short for typeglob, is a structure that has a slot for each variable type supported by Perl (and a few more). By using a glob for each symbol in the symbol table, we can have variable of each type with the same name.

• Scalar ($foo)
• Array (@foo)
• Hash (%foo)
• Sub (&foo)
• File handle
• Dir handle
• Format
• etc

$ftp is expected to be a glob. (Probably so it can be used as a file handle.)

${*$ftp}{net_ftp_host}
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$ftp->**->{net_ftp_host}
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$ftp->*{HASH}->{net_ftp_host}
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But it's not equivalent to

$ftp->{net_ftp_host}   # XXX
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The program is accessing the HASH slot of the glob, which contains a reference to a hash with the object's attributes.