Well the extended message that you quote seems clear enough in explaining that the earlier instance of the variable still exists and still takes up memory. You just can't access it. That would seem to me to be enough of a reason not to do this.

But you also need to think about what you're trying to do here. When you declare a variable, it should be for as small a scope as possible and each variable you declare should have only one use.

What are you really trying to achieve by redeclaring the variable? If you just want to put a new value into it then just overwrite the existing value. If you want to reset the variable to an undefined state then allocate undef to it. But if you're doing this to indicate that you're using the variable for a different purpose now, then that's a bad idea and you should be declaring a new variable with a different name.

So redeclaring a variable within a scope like this is a bad practice - partly because, yes, you are wasting a small amount of memory, but most importantly because it's unclear what you are trying to indicate by doing this and it is very likely to confuse your maintenance programmer.

daveorg,

Thank you for the very detailed answer! You provided a wonderful reply.

But if you're doing this to indicate that you're using the variable for a different purpose now, then that's a bad idea and you should be declaring a new variable with a different name.

That piece hit me between the eyes. Often, I'm using "my" to indicate a new purpose.

Thanks, Tatnall

"Recognizing who we aren't is only the first step toward knowing who we are." - Os Guinness

Simply put, you may be overwriting a variable that don't want to.

Consider:

use strict;
use warnings;

my $x = very_important_number_calculation();
printf "Your very important value is $x\n";

# time passes ...
my $x = temporary_value();
my $y = temporary_value();
printf "(x + y) = ($x + $y) = ", $x + $y;

# more time passes ...
print "Your very important number calculation yielded:  $x\n";
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See how that could ruin your day if you weren't warned about it?

There's a simple solution if you're sure you want to reuse the variable, though.  Just do it, but omit the my after the first time:

#!/usr/bin/perl
use strict;
use warnings;

my $pet = 'dog';
   $pet = 'cat';
   $pet = 'ferret';
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And I wouldn't call that "bad programming practice".  It's clear that you're intentionally resetting the variable to different values, and there's nothing (inherently) wrong with that.

You don't you just use *assingment* to clobber an old value?

my $pet = 'dog';
...
$pet = 'cat';
...
$pet = 'ferret';
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use strict;
use warnings;

my $a = 1;        # a simple variable
my $ref_a = \$a;  # refers to the original $a

my $a = 5;        # obscures the old $a, but
                  # ref_a still points to the old $a

print "'a' = $a; 'ref_a' points to ",  ${$ref_a}, "\n";
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Now of course you see what to expect, but in a larger program this behaviour can leave you puzzled. You should of course have created a new variable instead.

my $variable = "current value";

while ( whatever() ) {
   ...
   my $variable = "new value";  # if "my" was just typed by habit here
   ...
}

print $variable;  # then there's probably a bug when we get here
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