First of all, make the intent of your code clear. 

grep { s/\d+/a/ } @dd;

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should be 
@dd = map { s/\d+/a/r } @dd;

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or 
for (@dd) { s/\d+/a/; }

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or 
s/\d+/a/ for @dd;

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Unlike grep and do, eval makes a copy of the values returned like a sub call. 

$ perl -E'@dd = 0..9; s/\d+/a/ for @dd;                     say @dd;'
aaaaaaaaaa

$ perl -E'@dd = 0..9; s/\d+/a/ for grep { 1 } @dd;          say @dd;'
aaaaaaaaaa

$ perl -E'@dd = 0..9; s/\d+/a/ for do { @dd };              say @dd;'
aaaaaaaaaa

$ perl -E'@dd = 0..9; s/\d+/a/ for sub :lvalue { @dd }->(); say @dd;'
aaaaaaaaaa

$ perl -E'@dd = 0..9; s/\d+/a/ for sub { @dd }->();         say @dd;'
0123456789

$ perl -E'@dd = 0..9; s/\d+/a/ for map { $_ } @dd;          say @dd;'
0123456789

$ perl -E'@dd = 0..9; s/\d+/a/ for eval { @dd };            say @dd;'
0123456789

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In reply to Re: what difference between eval and do ? by ikegami
in thread what difference between eval and do ? by Anonymous Monk

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