Here's how to check for yourself.

$ perl -MBenchmark=cmpthese -e'my ($foo, $bar); cmpthese(10000000,{ in
+terp => sub{ $foo = qq(gabbit$/)}, concat => sub { $bar = q(gabbit).$
+/}});'
Benchmark: timing 10000000 iterations of concat, interp...
    concat:  6 wallclock secs ( 4.36 usr +  0.00 sys =  4.36 CPU) @ 22
+93577.98/s (n=10000000)
    interp:  6 wallclock secs ( 4.60 usr + -0.01 sys =  4.59 CPU) @ 21
+78649.24/s (n=10000000)
            Rate interp concat
interp 2178649/s     --    -5%
concat 2293578/s     5%     --
$
[download]

Not very significant, particularly since piecing together strings is usually preparation for I/O, a much slower proposition.

Update: A second run, careful to leave other processes idle, produced zero difference. That should be expected.

After Compline,
Zaxo