strict refs 

This generates a runtime error if you use symbolic references (see perlref).

...

There is one exception to this rule:
    $bar = \&{'foo'};
    &$bar;

is allowed so that goto &$AUTOLOAD would not break under stricture.

##</code><code>##

>perl -Mstrict -e "sub walk {} sub run {} my $walkable = defined(&{'walk'}); my $runnable = &{'run'};"
Can't use string ("run") as a subroutine ref while "strict refs" in use at -e line 1.