$ perl -We '$_="I have 10 dollars.\n"; s/(10)/$1/; print;'
I have 10 dollars.
$ perl -We '$_="I have 10 dollars.\n"; s/(10)/$1+1/; print;'
I have 10+1 dollars.
$ perl -We '$_="I have 10 dollars.\n"; s/(10)/$1+1/e; print;'
I have 11 dollars.

##</code><code>##

$ perl -We '$_="I have 10+4 dollars.\n"; s/(10\+4)/$1/; print;'
I have 10+4 dollars.
$ perl -We '$_="I have 10+4 dollars.\n"; s/(10\+4)/$1/e; print;'
I have 10+4 dollars.
$ perl -We '$_="I have 10+4 dollars.\n"; s/(10\+4)/$1/ee; print;'
I have 14 dollars.