Examine the $^O variable which will contain the name of the operating system:
print "$^O\n";
Which prints linux on Linux and MSWin32 on Windows.
You can also refer to this variable by the name $OSNAME if you use the English module:
use English qw' -no_match_vars ';
print "$OSNAME\n";
According to perlport, $^O will be darwin on Mac OS X.
You can also use the Config core module, which can provide the same information (and a lot more):
use Config;
print "$Config{osname}\n";
print "$Config{archname}\n";
Which on my Ubuntu machine prints:
linux
i486-linux-gnu-thread-multi
Note that this information is based on the system that Perl was built, which is not necessarily the system Perl is currently running on (the same is true for $^O and $OSNAME); the OS won’t likely be different but some information, like the architecture name, may very well be.