You code has only two print statements, but your output shows three lines; it's a little unclear whether you've inadvertantly omitted another print statement.

However, presuming that the missing code looks like these two lines:

$v += $v;
print "$v\n";
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I may have an explanation.

Which is, the increment and decrement operators, in particular, can be used to increment strings. In other words, they are not strictly numeric operations.

my $x = 'A';
++$x;
print "$x\n";
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would print

B

So, changing the ++ to += 1 in your example produces different results.

use warnings;
use strict;
my $v = 99;
$v = sprintf( "%04d", $v );
print "$v\n";
$v += 1;
print "$v\n";

$v += $v;
print "$v\n";
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prints:

0099
100
200
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dmm

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