• behind-the-scenes: doing $x++ when $x is undef returns 0, not undef, in a magical sort of way (the ++ causes it to return 0)
• highly advanced technology^*: $! is both a number and a string. printf "%d: %s", $! = 10, $!;

"Any sufficiently advanced technology is indistinguishable from magic"
- Arthur C Clarke

Generally, it means something incredibly clever and complex that the writer doesn't have time to explain in detail.

"Perl makes the fun jobs fun
and the boring jobs bearable" - me

magic

1. adj. As yet unexplained, or too complicated to explain; compare automagically and (Arthur C.) Clarke's Third Law: "Any sufficiently advanced technology is indistinguishable from magic." "TTY echoing is controlled by a large number of magic bits." "This routine magically computes the parity of an 8-bit byte in three instructions." 2. adj. Characteristic of something that works although no one really understands why (this is especially called black magic). 3. n. Stanford A feature not generally publicized that allows something otherwise impossible, or a feature formerly in that category but now unveiled. 4. n. The ultimate goal of all engineering & development, elegance in the extreme; from the first corollary to Clarke's Third Law: "Any technology distinguishable from magic is insufficiently advanced".

Parodies playing on these senses of the term abound; some have made their way into serious documentation, as when a MAGIC directive was described in the Control Card Reference for GCOS c.1978. For more about hackish `magic', see Appendix A. Compare black magic, wizardly, deep magic, heavy wizardry.

       Any SV may be magical, that is, it has special features
       that a normal SV does not have.  These features are stored
       in the SV structure in a linked list of "struct magic"'s,
       typedef'ed to "MAGIC".

An SV is Perl's internal representation of a scalar value. As I understand it, magic allows accesses (reads, writes) from/to variables to call special functions that do special things.

One simple example of this is the %ENV hash: changing values in %ENV change the environment for spawned subprocesses.

Another example of this is the %SIG hash.

Extensions (XS modules) can also add their own magic to variables.

Main Entry: 1mag·ic
Pronunciation: 'ma-jik
Function: noun
Etymology: Middle English magique, from Middle French, from Latin
magice, from Greek magikE, feminine of magikos Magian, magical, from
magos magus, sorcerer, of Iranian origin; akin to Old Persian magus
sorcerer
Date: 14th century
1 a : the use of means (as charms or spells) believed to have supernatural
power over natural forces b : magic rites or incantations
2 a : an extraordinary power or influence seemingly from a supernatural
source b : something that seems to cast a spell : ENCHANTMENT
3 : the art of producing illusions by sleight of hand 

That's magic!

jeffa hands boo some <Doug_Henning> tags

For instance, case sensitivity can be a gotcha for Windows users logging in to a UNIX webserver. Case insensitivity can be magic for UNIX users logging into a Windows webserver.