There is one time when requireing a module will limit it's scope. When using threads:

C:\test>perl -Mthreads 
-wle"async{require CGI; print 'T:'.%{CGI::}}->detach; sleep 2; print '
+M:'.%{CGI::};"

T:61/128
M:0
[download]

The above shows that a module required in a thread is not visible in other threads.

But if you use it anywhere, it will be visible (and consume space) in all threads:

C:\test>perl -Mthreads 
-wle"async{ use CGI; print 'T:'.%{CGI::} }->detach; sleep 2; print 'M:
+'.%{CGI::};"

T:62/128
M:62/128
[download]