$str = q{
1. This is just a sample.
2. This is to check 
3. How a set of values 
4. can be replaced by another 
5. set of values and that too 
6. in the most efficient way.
};

$str =~ s/^(\d)/chr($1+96)/meg;

print $str;

### prints:
a. This is just a sample.
b. This is to check 
c. How a set of values 
d. can be replaced by another 
e. set of values and that too 
f. in the most efficient way.