It depends on how often you would see it used. To start with put the sub in a file called something.lib You will not need the shebang line. At the end of the file have the last line be just 1; example:

sub my_silly_sub()
{
print "This is a silly sub";
}


1;
[download]

And in the perl scripts that use it use

require "something.lib";
[download]

Depending on the need to use it across lots of scripts or a few, you may than want to move its location to an area common to your scripts. But I wouldn't make it a module until you find it spreading. But than again there are many opinions on what to do. Choose what is best for you.