The issue only occurs when a specific multiply op (i.e. a particular '*' on a particular line) is called multiple times, and on one occasion returns an integer result, and on subsequent other occasions returns a float. When this occurs the PADTMP (the private variable that the op uses to return its result) gets upgraded from an SVt_IV to a SVt_PVNV, which is the smallest type that can hold both an integer and a float. As it happens it can also potentially hold a string, and because of this, it is more expensive to free. So the extra time you're seeing in the benchmarks is just due to freeing the temporary array's now-more-complex elements. You can see a similar effect here, which involves no arithmetic: 
use Benchmark 'cmpthese';

my $x = 1;
$x = 1.1; # $x promoted to PVNV
our @a = map $x, 1..1e6;

my $y = 1.1;
our @b = map $y, 1..1e6;

cmpthese( -2, {
    1 => '{my @c = @a}',
    2 => '{my @c = @b}',

__END__
    Rate    1    2
1 26.7/s   -- -35%
2 40.9/s  53%   --

[download]

As it happens, perl's multiply operator is optimised to handle int*int and float*float quickly; other permutations like int*float and float*string take slower paths. As it also happens, if you do $float * 4, the constant 4 is internally upgraded to hold both an IV and NV value, so subsequent iterations take the fast float*float code path.

Dave.

In reply to Re^4: Should multiplication by integer be avoided in favour of division for performance reasons? (benchmark pitfalls) by dave_the_m
in thread Should multiplication by integer be avoided in favour of division for performance reasons? by vr

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