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.