You can delete it from the symbol table: 
use strict;
use warnings;
sub foo {
    delete $::{foo};
    print "alive\n";
}
foo();
eval "foo()" or die $@;

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Note that I put the second call to foo into an eval because Perl looks up the symbol at compile type.
Perl 6 - links to (nearly) everything that is Perl 6.

In reply to Re: undef-ing an active subroutine by moritz
in thread undef-ing an active subroutine by perl5ever

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