Maybe the -P switch is what you want. You can use then the following construct:

#!/usr/bin/perl -P

#define MYPATH "/WEBDEVELOPMENT/cgi-bin/postcode.txt"

read_in_file();

sub read_in_file {
   open(INPUTFILE, "<".MYPATH."postcode.txt");
}
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That way you can define things at the top of your file and use the defined values through your code. See perlman:perlrun for more information about that switch.
--
Alfie