gurpreetsingh13,

I understand your reasoning, and for most languages you would be correct. But Perl is different: in Perl, the behaviour of functions and operators depends on context. For example, here is what the documentation says about backticks:

The collected standard output of the command is returned ... In scalar context, it comes back as a single (potentially multi-line) string, or undef if the command failed. In list context, returns a list of lines ..., or an empty list if the command failed.

So, in your script:

perl -e 'print chr(ord()+1) foreach(split //,`cat file.pl`);'
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the expression `cat file.pl` is in scalar context (because split expects a scalar expression here), so the output of the cat command is fed to split as a single, multi-line string. But in your other script:

perl -e '@lines=`cat file.pl`;print $#lines,"\n", $lines[2];'
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the assignment to an array (@lines) puts `cat file.pl` into list context, so the output of the cat command is here not a single string as before, but rather a list of strings (one for each line).

Context is a central concept in Perl. You can read up on it in the references given by Anonymous Monk, above, or in Chapter 2 of Programming Perl by Christiansen, foy, and Wall (in the section beginning on page 76 of the 4^th Edition).

Hope that helps,