my $x = '123'; 
(my $y = $x) =~ s/(2)/$1+1/e;
print $y;
[download]

Or

my $x = '123'; 
(my $y = $x) =~ tr/2/3/;
print $y
[download]

Or even ;)

my $x = '123'; 
my $y = $x + 10;
print $y
[download]