Exactly what the message says, a prototype mismatch

$ perl -Mdiagnostics -e " sub trim(); sub trim($){warn@_} ; "
Prototype mismatch: sub main::trim () vs ($) at -e line 1 (#1)
    (S prototype) The subroutine being declared or defined had previou
+sly been
    declared or defined with a different function prototype.
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