As for the actual internals, you would need to look at the Perl source. (shiver) --> perlguts

HTH,
SSF

$#array is the index of the last element in the array. Perl obviously knows how many elements there are in an array - if it didn't, then it wouldn't be able to automagically grow the array as you add more stuff to it and you'd have to handle memory allocation yourself. And perl obviously knows the index of the first element, which is the value in $[. Knowing how big the array is and the starting index, calculating the last index is trivial.

perl -le '@arr=qw(10 1 2 3 4);$[=3;print $arr[$_],"\t",$_ for 0 .. $#a
+rr;'
10    0
3    1
4    2
10    3
1    4
2    5
3    6
4    7
[download]

From perlvar:

$[

The index of the first element in an array, and of the first character
+ in a 
substring. Default is 0, but you could theoretically set it to 1 to ma
+ke Perl 
behave more like awk (or Fortran) when subscripting and when evaluatin
+g the 
index() and substr() functions. (Mnemonic: [ begins subscripts.)

As of release 5 of Perl, assignment to $[ is treated as a compiler dir
+ective, and 
cannot influence the behavior of any other file. (That's why you can o
+nly assign 
compile-time constants to it.) Its use is deprecated, and by default w
+ill trigger 
a warning.

Note that, unlike other compile-time directives (such as strict), assi
+gnment to 
$[ can be seen from outer lexical scopes in the same file. However, yo
+u can use 
local() on it to strictly bind its value to a lexical block.
[download]

I suspect you can assign 1 to it, but other values are silently ignored. I tried your code with $[=1, and lo and behold:

perl -le '@arr=qw(10 1 2 3 4);$[=1;print $arr[$_],"\t",$_ for 0 .. $#a
+rr;'
10    0
10    1
1    2
2    3
3    4
4    5
[download]

(shrug)

SSF

How internally array holds the value like this way.I have checked the code by using Dumper function in Data::Dumper module.
In that, it printed only 10,1,2,3,4 but when I loop through the array.It printed 6 values as 10,10,1,2,3,4.Explain me how it works?
Thanks in advance

I suspect you can assign 1 to it, but other values are silently ignored

You suspect incorrectly.

$ perl -e '@array=(qw(ant bat cat)); $[ = 94; print $array[95]."\n"'
bat

$ perl -e '@array=(qw(ant bat cat)); $[ = -9; print $array[-8]."\n"'
bat
[download]

Please explain me about how $#array is internally working.

The size of the array, plus the value of $[ is enough to calculate the size of $#array.

Illustrated details.

Actually, because $#array can be used as an lvalue, a fair bit of magic is going on (including adding "magic" to both the returned scalar and the array in some circumstances). Fortunately, it is invisible to the user.
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