sub foo { print( "Hello World\n" ); }

BEGIN { *bar = \&foo; }

foo();
bar();
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The BEGIN is not necessary, but it can avoid problems.

Update: Or to demonstrate your requirements:

sub foo { print( "foo\n" ); }

BEGIN {
   *bar = \&foo;
}

BEGIN {
   *foo = sub { print( "bar\n" ); };
}

foo();  # bar
bar();  # foo
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