For conditional a use statement, see the if module. But if you're happy to include your modules at runtime then a simple require will be be. Finally, to check for the existence of a subroutine you can simply use defined e.g

defined &mysubtocheck && mysubtocheck();
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Although make sure you use parens when calling a dynamically defined subroutine otherwise you'll get a compile-time error under strict 'subs'.