It has to be a scalar, but it doesn't need to be a variable. You can say 
  $f->${\("prefix_$key")}($atr);

[download]
However, using a variable is probably clearer.

Update: added the missing '$' pointed out by ikegami

--Dave
Opinions my own; statements of fact may be in error.

In reply to Re^2: Dynamically constructed function calls by dpuu
in thread Dynamically constructed function calls by WalruZ

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