> perl -le 'print ( undef == 0 ? "yes" : "no" )'
yes

> perl -le 'print ( undef == "0" ? "yes" : "no" )'
yes

> perl -le 'print ( undef == "" ? "yes" : "no" )'
yes

> perl -le 'print ( undef eq 0 ? "yes" : "no" )'
no

> perl -le 'print ( undef eq "0" ? "yes" : "no" )'
no

> perl -le 'print ( undef eq "" ? "yes" : "no" )'
yes