I'm really puzzled. Recently when reading through the perlref article, I came across a little example of closures. The example first confused me, and then made more sense, and then confused me again. Here is how.

At first, I was confused because, according to my thinking, each of the generated functions should print out the same value: <FONT COLOR='violet'>....</FONT>. After all, 'violet' is the last known value of $name.. Here is the example:

for my $name (@colors) {
  no strict 'refs';    # allow symbol table manipulation
  *$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
}
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Then it occured to me that I didn't look carefully enough: $name was local to the loop (it is declared for my $name ....) so maybe the value set by one loop iteration didn't affect the value in the next. Problem solved?

Well, no. To test this idea out I decided to experiment a bit by moving the declaration to outside of the for loop. Just for kicks I did this using both my and our to declare it. This resulted in some odd behavior that I'm having trouble understanding. When our is used to declare $name the closure sometimes complains of an undefined variable. When my is used to declare $name, perl sometimes refuses to change the value of the variable. Is this normal? is it a bug? I would be grateful to any fellow monks that can help me form a mental model to explain this behavior. I don't feel I understand this issue well enough to make a judgment either way.

I've put together an annotated script with some examples:

use strict;
use warnings;

# declaration of $name
# pick one and comment out the other
my $name;
#our $name;

# why is red printing out red rather than violet since
# $name is always changing? Shouldn't it be picking up the
# most recent value of $name, i.e. 'violet'?

my @colors = qw(red blue green yellow orange purple violet);
for $name (@colors) {
  no strict 'refs';    # allow symbol table manipulation
  *$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
}

# And another oddity: when we declare <c>our $name</c> *both*
# red and violet complain about uniinitialized values even
# though we have clearly set <c>$name</c> inside the for loop.
# No such problems when we declare <c>$name</c> using <c>my</c>.

print "----- \$name is constant inside sub ------\n";
print "red:    ${\(red())}\n";
print "violet: ${\(violet())}\n";


# is it because our lack of use of $name within the generated
# sub caused it to be compiled as a constant?
# no: here we use $name, but it *still* prints out the value
# at sub definition time, rather than the current value.

for $name (@colors) {
  no strict 'refs';       # allow symbol table manipulation
  no warnings 'redefine';  #ignore noise about redefinitions
  *$name = sub {
    my $sOutput = "<FONT COLOR='$name'>@_</FONT>";
    $name = 'something wacky and wonderful'; #change the value
    return $sOutput;
  };
}

# when we declare <c>our $name</c> red, but not violet
# complains about uninitialized values.  There are no
# complains when we declare <c>my $name</c>. Hmmm...

print "----- \$name given new value inside sub ------\n";
print "red:    ${\(red())}\n";
print "violet: ${\(violet())}\n";


# so maybe the closure picks up the value at the time of
# definition and always uses that as the initial value even
# if the value changes after subdefinition?
#
# but if so, why do red and violet use their own last value
# setting of the value of name? Didn't we just redefine the
# subroutines? shouldn't they be using the value at the time
# of definition?

print "----- sub used inside loop that defined it ------\n";
for $name (@colors) {
  no strict 'refs';       # allow symbol table manipulation
  no warnings 'redefine';  #ignore noise about redefinitions

  # why isn't $name getting set to the array eleemnt value
  # when the array element is red or violet?
  # Note: it gets set properly when we declare
  # <c>our $name</c> but not when we declare <c>my $name</c>

  print "trying out $name: ";
  *$name = sub {
    my $sOutput = "<FONT COLOR='$name'>@_</FONT>";
    $name = 'something wacky and wonderful'; #change the value
    return $sOutput;
  };
  print $name->() . "\n";
}

print "----- sub used outside of loop that assigned it ------\n";
print "red:    ${\(red())}\n";
print "green:  ${\(green())}\n";
print "violet: ${\(violet())}\n";
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Here is the output generated when $name is declared using my $name.

----- $name is constant inside sub ------
red:    <FONT COLOR='red'></FONT>
violet: <FONT COLOR='violet'></FONT>
----- $name given new value inside sub ------
red:    <FONT COLOR='red'></FONT>
violet: <FONT COLOR='violet'></FONT>
----- sub used inside loop that defined it ------
trying out something wacky and wonderful: <FONT COLOR='something wacky
+ and wonderful'></FONT>
trying out blue: <FONT COLOR='blue'></FONT>
trying out green: <FONT COLOR='green'></FONT>
trying out yellow: <FONT COLOR='yellow'></FONT>
trying out orange: <FONT COLOR='orange'></FONT>
trying out purple: <FONT COLOR='purple'></FONT>
trying out something wacky and wonderful: <FONT COLOR='something wacky
+ and wonderful'></FONT>
----- sub used outside of loop that assigned it ------
red:    <FONT COLOR='something wacky and wonderful'></FONT>
green:  <FONT COLOR='something wacky and wonderful'></FONT>
violet: <FONT COLOR='something wacky and wonderful'></FONT>
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And here is the same script run when my $name is commented out and replaced by our $name.

----- $name is constant inside sub ------
Use of uninitialized value in concatenation (.) or string at ...
red:    <FONT COLOR=''></FONT>
Use of uninitialized value in concatenation (.) or string at ...
violet: <FONT COLOR=''></FONT>
----- $name given new value inside sub ------
Use of uninitialized value in concatenation (.) or string at ...
red:    <FONT COLOR=''></FONT>
violet: <FONT COLOR='something wacky and wonderful'></FONT>
----- sub used inside loop that defined it ------
trying out red: <FONT COLOR='red'></FONT>
trying out blue: <FONT COLOR='blue'></FONT>
trying out green: <FONT COLOR='green'></FONT>
trying out yellow: <FONT COLOR='yellow'></FONT>
trying out orange: <FONT COLOR='orange'></FONT>
trying out purple: <FONT COLOR='purple'></FONT>
trying out violet: <FONT COLOR='violet'></FONT>
----- sub used outside of loop that assigned it ------
red:    <FONT COLOR='something wacky and wonderful'></FONT>
green:  <FONT COLOR='something wacky and wonderful'></FONT>
violet: <FONT COLOR='something wacky and wonderful'></FONT>
[download]

Many thanks in advance, beth