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.