All the indexes of array starts with 0.How to change the start index of array to "101";

Subtract 101 from the index.

use constant BASE => 101;
$a[$i - BASE] = 'a';
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This will still work:

print("The array has ", 0+@a, " elements\n");
for (@a) {
    ...
}
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for my $i (101..200) {
    ... $a[$i - BASE] ...
}
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There's also $[, but

• It affects all arrays
• It affects substr
• It's used is discouraged.
• It will issue a deprecation warning by default in Perl 5.12.
• It will be removed in Perl 5.14.

I'd avoid it.

How to see the documentation of perl modules installed in Unix box

perldoc or online on CPAN

Update: Compensated for a warning. There needs to be space between - and BASE.