You are correct, at least according to the B::Deparse documentation. Interpolation is just syntactic sugar for concatenation. You can see it for yourself:

$ perl -MO=Deparse,-p,-q -e '$foo = "BAR"; $bar = "a${foo}c"; $baz="a"
+.$foo."c"'                        
($foo = 'BAR');
($bar = (('a' . $foo) . 'c'));
($baz = (('a' . $foo) . 'c'));
[download]

Update: Added concatenation to the example for comparison. Also, the output using B::Terse to dump the opcode tree may be more convincing.

So, in the spirit of further useless benchmarking:

use Benchmark;

timethese( 10000000, {
  5 => sub {my $R = 5;},
  6 => sub {my $R = 6;}
});
[download]

    5:  6 wallclock secs ( 6.61 usr + -0.03 sys =  6.58 CPU) @ 1519756
+.84/s
    6:  6 wallclock secs ( 5.89 usr +  0.08 sys =  5.97 CPU) @ 1675041
+.88/s
[download]

Russ
Brainbench 'Most Valuable Professional' for Perl