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use Benchmark;

...
  interp => sub {my $R = 1; my $S = "Test${R}a"},
  append => sub {my $R = 1; my $S = 'Test' . $R . 'a'}
});

Benchmark: timing 1000000 iterations of interp, manual...
  interp:  4 wallclock secs ( 3.68 usr +  0.05 sys =  3.73 CPU) @ 2680
+96.51/s
  manual:  3 wallclock secs ( 3.79 usr + -0.13 sys =  3.66 CPU) @ 2732
+24.04/s