I realize this is an old post, but I just came from 423779...
A closure is subroutine that has captured its lexical environment which has now gone out of scope.
For completeness, I would add that not all closures are subroutines; regular expressions that use the (?{}) construct can also be closures. For example, 
sub a { 
  my $i = 0; 
  # note that the regex is compiled only once
  () = shift =~ /a(?{print $i++})/g;
} 
a("ababa"); # prints 0 1 2
a("ajaja"); # prints 3 4 5

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I don't know if these regular expressions are implemented internally as subroutines, but they certainly don't look like subroutines, so people are often surprised to see this behavior.

In reply to Re: •closures are NOT anonymous subroutines necessarily by itub
in thread subs as args by Anonymous Monk

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