One of the purposes of & (in 5.8.x) is to force the interpreter to use your own function. However, I've only seen examples of it overriding builtins with subs defined in the same script, not imported. Maybe

use Foo;
...
my $wuzzie = &Foo::sysread(FH, $scalar, $length);
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would do it?
Or perhaps just &sysread(FH, $scalar, $length).
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