DB<1> @a= qw/zero one two three four/

 DB<2> $pi ='three . one four one'

 DB<3> $pi =~ s/$a[$_]/$_/ig for 0..4

 DB<4> p $pi
3 . 1 4 1
 DB<5>
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Please note that replacing all the "one" before replacing "two" might change the input in unexpected ways.

It really depends if you are really replacing numbers ...