#!/usr/bin/perl -l

sub this { 'this' }
print this;
undef &main::this; # undefine the code slot
print main->can('this') ? 'main->can("this")' : '! main->can("this")';
eval {print this};
print $@;
undef *main::this; # undefine the entire glob
print main->can('this') ? 'main->can("this")' : '! main->can("this")';

__END__
# result
this
main->can("this")
Undefined subroutine &main::this called at sub.pl line 7.

! main->can("this")