When you declare a variable with my it won't be accessible in require-d code. One alternative is to use our instead of my: 
our $var;
require 'library.pl';
# library.pl contains:
#   $var = ...
print $var, "\n";

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However, modifying variables this way in not a great idea. Think about what the purpose of the module is. For instance, if you are using it to compute a function, make the module define a subroutine which is called by your program when it's needed. If is being used to define constants, then consider putting it in its own namespace and exporting the values.

In reply to Re: use, require, do or what? by pc88mxer
in thread use, require, do or what? by AKSHUN

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