sub x{ my( $n ) = @_; $n };;

cmpthese -1, {
    a => q[ my $n; $n += $_    for 1 .. 1e5 ], 
    b => q[ my $n; $n += x($_) for 1 .. 1e5 ] 
};;

    Rate    b    a
b 2.14/s   -- -80%
a 10.7/s 399%   --