Because you are getting the return value of the substitution, instead of the changed text. Try adding the -r flag, introduced in Perl 5.14:

my @values = map { s/dc=domain,dc=tld/dc=sub,dc=otherdomain,dc=tld/ir 
+} @$array_ref;
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perl -E '
@ary =( "abc", "cbc", "dbd" ); say for map { s/b/FOO/r } @ary;
'
aFOOc
cFOOc
dFOOd
[download]