#!/Perl question

[Hans van den Boogert <hansfong@mail.geocities.com>]

I'm trying to learn some Perl. I found an on-line book which is Unix
biased, but the scripts all start with #!/usr/local/bin/perl (which makes
sense if you have compiled and installed it yourself). However, on my
Debian system Perl was of course put in /usr/bin/perl. So how can I write a
Perl script that be executed on a variety of systems (Unix/Linux/DOS)?? 

-- a somebody who last programmed in BASIC back in '84. (Please don't
laugh. I was young and my teacher made me to).