when a variable is declared in the main block only, why do we neeed to make it local ?

• To ensure it's only used in the "main block". In other words, it provides encapsulation.
• To allow the program to run. It won't run if you don't (assuming you didn't unwisely disable strictures by omitting use strict;)

If a variable is declared local in the main block, does it mean that if a subroutine is declared then this variable can not be used inside subroutines as its declared outside the subroutine as local ?

It depends on the ordering, since the subroutines exist in the file's lexical scope (what you call "the main block").

use strict;

my $x;

sub foo {
   $x = 'foo';   # Ok
}

foo();
print($x);  # foo
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-vs-

use strict;

sub foo {
   $x = 'foo';   # Refers to package var $main::x.
                 # It's also a compilation error.
}

my $x;
foo();
print($x);  # foo
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my $dirname = dirname $old_name ; # what does this syntax mean ?

Call sub dirname with argument $old_name. Declare $dirname and assign the result of the call to dirname to it.

s/^/not/, # does it mean that not will be added at the start of the string ?

Yes. Why didn't you try it?

/^/ in a regexp patterm means "the start of the string" (or "the start of the line" if the "m" modifier is used). So s/^/not/ means "replace the start of the string with 'not'" or in better English, "prepend 'not' to the string". It could have been written

$basename = 'not' . $basename;
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(my $basename = $name =~ s#./##; # how come there is only one forward slash ?

Why not?

Though that's buggy for two reasons. There's a missing closing paren, and it should be s#.*/##s ("replace everything up to the last slash with nothing").