The /e switch treats the replacement as an expression to be executed. This is not the same as evaluating the contents of $1..$n as executable code, i.e. treating the result of the interpolation as executable code, which needs "double eval" (/ee).

You could re-write

s/(10\+4)/$1/ee;
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as

s/(10\+4)/eval $1/e;
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In the case of s/(10\+4)/$1/e the replacement just resolves as the code line

"10+4";
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which yields a string upon execution.

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