Shell Programming and Scripting

first.key ?{x.y.z}
second.key ?{xa.ys.zz.s}
third.key ?{xa.k}

first.key ?{x_y_z}
second.key ?{xa_ys_zz_s}
third.key ?{xa_k}

awk -v FS='[}{]' '{gsub(/\./,"_",$2); print $1"{"$2"}"}' /path/of/file

$ cat data
first.key ?{x.y.z}
second.key ?{xa.ys.zz.s}
third.key ?{xa.k}
$ sed -e :a -e's/\([^{][^{]*{[^.][^.]*\)\./\1_/;ta' < data
first.key ?{x_y_z}
second.key ?{xa_ys_zz_s}
third.key ?{xa_k}
$
$

sed -e :a -e's/\([^{]\+{[^.]\+\)\./\1_/;ta' < data

$ cat file
first.key ?{x.y.z}
second.key ?{xa.ys.zz.s}
third.key ?{xa.k}
$ perl -pe's/([^.]+)\.(?!\w+\s)/$1_/g' file
first.key ?{x_y_z}
second.key ?{xa_ys_zz_s}
third.key ?{xa_k}