Along with the lovely suggestions from ikegami and moritz, you can use caller to make the deletion less fragile if you move your subroutine about:

#!/usr/bin/perl
use strict;
use warnings;

foo();
eval{foo()} or warn $@;
print "Done\n";

sub foo {
    {
        no strict 'refs';
        undef *{(caller(0))[3]};
    }
    print "foo was called\n";
}
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