Call the function properly and it works. Note parentheses.

 
use strict; 
use warnings;

print func();

sub func
{
   return "123";
}
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Also I have added use strict; and use warnings; which is highly recommended and would have given you valuable feedback as to what the problem was.

Unquoted string "func" may clash with future reserved word ...
print() on unopened filehandle func ...
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