defined
Tests whether a value is defined (not undef).
Pops one value from the stack and pushes 1 if it is defined, 0
otherwise. For code values (Cv), “defined” means the CV has
actual executable code and is not merely an empty stub. For all
other types, delegates to Sv::is_defined().
Synopsis
if (defined $x) { ... }
if (defined &func) { ... } # is sub defined (not a stub)?
if (defined $hash{$k}) { ... }