[Omaha.pm] ignoring undefined subroutines

[Terry]

I want to ignore undefined subroutines.  Right now, my collection of
text ( not worthy of being called a perl script ) calls subroutines
but I want it to NOT die when it comes across a subroutine that is not
defined.  How can I do this?   For example:

&{"header_$who"};

if $who is not defined or contains a goofy value, my script dies, I do
not want this.

Thanks!
Terry