From perldoc perlop
Otherwise, the right side is a method name or a simple scalar variable containing either the method name or a subroutine reference, and the left side must be either an object (a blessed reference) or a class name (that is, a package name). See perlobj.

This means that you can't have a string or an expression as a method name. It has to be a simple scalar. So if you do the following, everything should work fine: 

my $method = "prefix_$key";
$f->$method($atr);

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In reply to Re: Dynamically constructed function calls by itub
in thread Dynamically constructed function calls by WalruZ

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