1. You might be, depending on your reasons for doing it.
2. There are a couple ways.

One possibility is to alias the symbol from one package to another.

package Pileofrogs::Util;
*{"Pileofrogs::Frob::frob"} = \*{"Pileofrogs::Util::frob"};

sub frob { 
    print "frob!";
}

package main;
Pileofrogs::Frob->frob;
Pileofrogs::Util->frob;
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Another way is to just change the original sub to one that calls the new one. You can also use this to emit a warning.

package Pileofrogs::Frob;
use Carp 'carp';

sub frob { 
    carp "You should really use Pileofrogs::Util::frob";
    Pileofrogs::Util->frob( @_ );
}
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