That this is an OS artifact can also be seen by being explicit about the numbers of all the lines at which print statements are executed (note that on my machine, STDOUT and STDERR have not (yet) gotten out of sync):

use strict;
use warnings;

my ($a, $b) = ("1_334", 1334);
printf "line %ld: $a $b \n", __LINE__;

$a += 1;
$b += 1;
printf "line %ld: $a $b \n", __LINE__;

printf "line %ld: Unequal \n", __LINE__ if ($a != $b);
printf "line %ld: $a $b \n", __LINE__;
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Output:

C:\@Work\Perl\monks\dilpane>perl ubar_in_qqted_n.pl
line 39: 1_334 1334
Argument "1_334" isn't numeric in addition (+) at ubar_in_qqted_n.pl l
+ine 41.
line 43: 2 1335
line 45: Unequal
line 46: 2 1335
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