If you declare a sub foo() in your perl code, and core::foo already exists, you will get a warning "Ambiguous call ....

Yes - but only if there's a call to foo() made after that declaration of sub foo().
At least, that's what I'm seeing on perl-5.38.0:

D:\>perl -wle "sub sqrt {return sprintf '%.4g', $_[0] ** 0.5}"

D:\>perl -wle "print sqrt(3); sub sqrt {return sprintf '%.4g', $_[0] *
+* 0.5};"
1.73205080756888

D:\>perl -wle "print sqrt(3); sub sqrt {return sprintf '%.4g', $_[0] *
+* 0.5}; print sqrt(2);"
Ambiguous call resolved as CORE::sqrt(), qualify as such or use & at -
+e line 1.
1.73205080756888
1.4142135623731
[download]

Cheers,
Rob