This is an interesting coincidence, I was just about to post a meditation regarding a problem that I've just this minute figured out.

use strict;
use warnings;

my %const;

#Initialise params
$const{pid}                = 1;
$const{pidwithzeros}     = '0' x (10 - length $const{pid}) . $const{pi
+d },

#Check params
die "Invalid PID $const{pid}\n"     if not $const{pid} =~ /^[0-9]+$/;

#Various lines of code removed

print $const{pidwithzeros}, "\n";
[download]

I was getting an "uninitialized value in print" warning on the final line. The mistake is obviously that the comma at the end of line 8 should be a semi-colon but for the life of me I couldn't see it. To try and find out what was going on I changed the code to the following but this only deepened my confusion:

use strict;
use warnings;

my %const;

#Initialise params
$const{pid}                = 1;
$const{pidwithzeros}     = '0' x (10 - length $const{pid}) . $const{pi
+d },

#Debug line
print $const{pidwithzeros}, "\n";

#Check params
die "Invalid PID $const{pid}\n"     if not $const{pid} =~ /^[0-9]+$/;

#Various lines of code removed

print $const{pidwithzeros}, "\n";
[download]

With the debug line in the code worked as expected. Without and the error would appear. Confusion reigned. Half an hour of thinking I was going bonkers later and as I was preparing a post to SoPW, I saw it.

Hey hum.