Re: Initialization of hash element....

Because of this I thought that if a hash element has no value and it is used on an arithmetic operation then its default value is zero. Am I correct?

Yes, and this has not strictly to do with hash elements, but with any variable, be it a simple scalar or an element of an aggregate. One caveat though: under warnings, you will get a warning if you use such a value in a generic expression, but you won't if you use mutators instead:

tilde:~ [13:53:30]$ perl -we '$a=$a+1'
Use of uninitialized value in addition (+) at -e line 1.
tilde:~ [13:53:50]$ perl -we '$a+=1'
Name "main::a" used only once: possible typo at -e line 1.
tilde:~ [13:53:58]$ perl -we '$a++'
Name "main::a" used only once: possible typo at -e line 1.
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Re^2: Initialization of hash element.... by ysth (Canon) on Jun 01, 2007 at 18:30 UTC
`$ perl -we '$a=0' Name "main::a" used only once: possible typo at -e line 1. Use of uninitialized value in multiplication () at -e line 1.` [download] Only certain mutators suppress the warning - those that most make sense with a 0 / "" default. These are: ++ and -- (either pre or post), +=, -=, .=, \|=, ^=, &&=, \|\|=. But last I checked, some of those erroneously warn when used on a tied variable.	[reply] [d/l]

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Re^2: Initialization of hash element....
by ysth (Canon) on Jun 01, 2007 at 18:30 UTC

$ perl -we '$a*=0'
Name "main::a" used only once: possible typo at -e line 1.
Use of uninitialized value in multiplication (*) at -e line 1.
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++ and -- (either pre or post), +=, -=, .=, |=, ^=, &&=, ||=.

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