use feature 'say';
undef &say;
*say = \&not_say;
say 'test';
sub not_say {
    print 'not saying';
}
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The confounding thing is that this works:

use strict;
use warnings;
use 5.012;

#Rule: sub names are entered into the symbol table.
sub abc {
    print "abc\n";
}

sub xyz {
    print "xyz\n";
}

local *abc; #gets rid of 'redefined main::abc' warning'
*abc = \&xyz;
abc;

--output:--
xyz
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But this doesn't work:

use strict;
use warnings;
use 5.012;

sub xyz {
    print "xyz\n";
}

local *say;
*say = \&xyz;
say 'hello';

--output:--
hello
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Nor does this:

use strict;
use warnings;
use 5.012;

use subs qw( say );  #Supposedly overrides a built in

my $verbose = 1;

sub say {
    if ($verbose) {
        print shift, " world\n";
    }
}

say 'hello';

--output:--
hello
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