undefined в perl

$txt=undef;

perldoc -f undef

[vilfred@observ contur]$ perl -e '$t=1; $t=undef; print "ok\n" unless $t'
ok
[vilfred@observ contur]$ perl -e '$t=1; undef $t; print "ok\n" unless $t'
ok
[vilfred@observ contur]$