Use require (which expects a namespace as a bareword or a path as an expression, and happens at run-time), not use (which expects a namespace as a bareword, and happens at compile time):

foreach $plugin (<Main/*.pm>) {
    require $plugin;
}
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Of course, I'm assuming the module really is a module (i.e. has a package). If it doesn't, you should be using do (and shouldn't use .pm).

I'm also assuming you're not trying to import any symbols, since use imports symbols, while require doesn't. I consider importing symbols from a dynamically loaded modules to be dangerous and unnecessary.