Simple and innocent looking backticks become a nightmare once you write code for more than one system. Some problems, some general, some with your example:

• You have to work with an unknown shell. Many shells behave according to POSIX, others don't. You don't know what shell you get. You can't write reliable code this way.
• You don't know the quoting rules of the shell, because you don't know the shell. You can't reliably quote program name and program arguments. You don't even know WHEN to quote names and arguments. On Windows, things are even worse. You may end passing invalid arguments to the invoked program.
• You don't know the I/O redirection rules of the shell, because you don't know the shell. < and > are pretty universal, but 2> isn't supported everywhere (e.g. command.com doesn't know that). Don't even think about 2>&1.
• /proc/cpuinfo is Linux specific. For example, FreeBSD doesn't have /proc/cpuinfo. Don't even think about Windows.
• cat is called without a path, so you will execute the first cat program found in $ENV{'PATH'}. Maybe you execute /usr/bin/cat, the program that concatenates files and prints them to STDOUT. Maybe you execute /home/evil/hoho/cat, a program that changes the root password and spawns a telnet daemon on port 37331 before pretending to be the innocent /usr/bin/cat. No big problem as long as you invoke that cat as unprivileged user, but when run as root, you loose.
• There is no cat on a fresh Windows system. A similar command is called type and is a shell built-in. Remember that it has a different behaviour (it adds output lines, both empty and with filenames, when called with more than one argument).
• There is no need to use cat inside backticks to read a file into a scalar: open my $h,'<','/proc/cpuinfo' or die "Could not read /proc/cpuinfo: $!";my $result=do { local $/=<$h> }; close $h;.
• Neither is there a need to use cat inside backticks to read a file into an array: open my $h,'<','/proc/cpuinfo' or die "Could not read /proc/cpuinfo: $!";my @result=<$h>; close $h;.

Alexander