Modern::Perl means strict. Under strict, every variable must be declared. $input is not declared. See also my.

Update: Try also diagnostics or splain:

$ echo 'Global symbol "$input" requires explicit package name at -e li
+ne 1.' | splain
Global symbol "$input" requires explicit package name at -e line 1 (#1
+)
    (F) You've said "use strict" or "use strict vars", which indicates
+ 
    that all variables must either be lexically scoped (using "my" or 
+"state"), 
    declared beforehand using "our", or explicitly qualified to say 
    which package the global variable is in (using "::").
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