No, $1something isn't ambiguous, $<digit> variables never are:

$ perl -le '$_ = "aaa\@z.com bbb\@z.com ccc\@z.com"; s/(.+?\@)(.+?\.)(
+.+?)/$1something.$3/g;print'
aaa@something.com bbb@something.com ccc@something.com
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In fact, a variable $1something is illegal:

$ perl -e '$1something'
Bareword found where operator expected at -e line 1, near "$1something
+"
        (Missing operator before something?)
syntax error at -e line 1, next token ???
Execution of -e aborted due to compilation errors.
[download]

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}