What the output is ?

I'll skip this one - it takes nothing to put the text inside a text file, and execute it using the Perl interpreter. perlrun can help you with this, anyway.

Explanation ?

For regex in general, the main reference is perlre, but a look to perlretut is worth a look as well. In your case, perlop is also a good starting point, in particular the "Regexp Quote-Like Operators".

In general, the "s/SOMETHING/OTHER/" construct makes substitutions. Wherever it finds "SOMETHING", it puts "OTHER". In this case, this substitution operator is applied to the $value variable, via the =~ binding operator. This can be found in perlop, of course.

The "e" switch indicates that the "OTHER" (i.e. the 'pack("C", hex($1))' in your case) is to be considered an expression and not a pure sequence of characters, and the "g" switch indicates that the substitution has to be applied to every occurence of the text matched in the first "SOMETHING" part (i.e. the '%(..)' in your case). Globally, this seems born to replace every occurrence of "%XX" with the character whose hexadecimal representation is "XX".

Could any one help me .......

A whole lot is willing to help you here. But please take a look around, and in particular to the About the PerlMonks FAQ and How do I post a question effectively?. You forgot to use proper code tagging, and forced a janitor to do it for you. Welcome to the Monastery!