require and therefore use execute the specified module just like a script. They expect the specified module to return a true value when executed. The return value of a module is the value returned by the last expression evaluated when the module is executed (just like for subs). 1 is a true value. You could change 1 to "A true value to satisfy require and/or use" (which is also a true value), and it would work just as well.

This might be best explained by showing what happens when 1 is changed to 0:

>type Module.pm
package Module;
# ...
0;

>perl -e"use Module;"
Module.pm did not return a true value at -e line 1.
BEGIN failed--compilation aborted at -e line 1.
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