As petral pointed out, dd outputs its status messages on STDERR, not on STDOUT, but open and backticks only capture STDOUT. You can get around this by redirecting STDERR into the STDOUT file handle; the syntax is the same as on the shell command line, because Perl calls the Bourne shell to set up the redirection:
dd if=infile of=outfile 2>&1 Example Perl code:

my @r = `dd if=infile of=outfile 2>&1`;
foreach (@r) {
    chomp;
    print "***$_***\n";
}
[download]

My dd on Linux does not output transfer speeds, just Records+PartialRecords in and out, so the program above printed:

***0+1 records in***
***0+1 records out***
[download]

Note that this only works correctly when you use the of parameter of dd. If you redirect STDOUT using the shell instead, then Perl will not see the status messages, because they get embedded somewhere in the output file. For example:

# BAD CODE
my @r = `dd if=infile >outfile 2>&1`;
[download]

I wrote this to give a dd specific example. In the perlfaq pointer (How can I capture STDERR from an external command?) that grep gave earlier, all the permutations of STDOUT vs. STDERR and open vs. backticks are discussed.

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