The way I see it, ${ EXPR1 } forces scalar context and then performs a scalar dereference on EXPR1.
The backslash \( EXPR2 ) forces list context on EXPR2 and returns a list of references.
By combining the two, you have \foo() returning a list of one reference to scalar 10. Then, ${ (\10) } returns the scalar dereferenced 10.
FYI, there's a catch to using the \( ) syntax for @arrays:
> perl -de ''
DB<1> x @array = (1 .. 3)
0 1
1 2
2 3
DB<2> x \(@array, (@array))
0 ARRAY(0x844cad8)
0 1
1 2
2 3
1 SCALAR(0x844e420)
-> 1
2 SCALAR(0x844ca3c)
-> 2
3 SCALAR(0x844cb80)
-> 3
DB<3> x \(@array)
0 SCALAR(0x844e420)
-> 1
1 SCALAR(0x844ca3c)
-> 2
2 SCALAR(0x844cb80)
-> 3
DB<4> x \(@array, ())
0 ARRAY(0x844cad8)
0 1
1 2
2 3
Update: BTW, same catch applies to %hashes.
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