sub rtn_closure {
  my $i = shift;
  return sub { $i++ };
}

# f's i starts at 5
my $f = rtn_closure(5);
print $f->(),"\n";  # Prints 5
print $f->(), "\n"; # Prints 6

# g gets a separate i that starts at 10
my $g = rtn_closure(10);
print $g->(), "\n"; # Prints 10
print $g->(), "\n"; # Prints 11
print $f->(), "\n"; # Prints 7
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sub exec_callback {
  my $f = shift;
  $f->();
}

exec_callback(sub {print "hello\n"});
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No, a callback need not be closed over any variables. A closure may be called by any means a normal function is.

Do you have a sample code showing the difference between the two?