I needed this for a crossplatform project. I found no documentation of the OS strings, so I did a bit of testing and ended up with this:
if ($^O =~ /mswin/i) {# Windows code}
elsif ($^O =~ /linux/i) {# linux code}
elsif ($^O =~ /darwin/i){# mac code}

[download]

This is designed to introduce some tolerance just in case some old or weird OS or perl versions produce slightly different OS strings. Seems to work fine so far.

In fact, I even added a fallback in case the OS string doesn't match any of these (let the user decide): 
my $OS;
if ($^O =~ /mswin/i) {$OS = "Windows";print "OS detected: Windows\n"}
elsif ($^O =~ /linux/i) {$OS = "Linux";print "OS detected: Linux\n"}
elsif ($^O =~ /darwin/i) {$OS = "Mac";print "OS detected: Mac OS X\n"}
+ 
else {print "\nUnable to detect OS type, choose your OS:\n\nWindows   
+ Any version of Microsoft Windows\nMac    Any flavour of Mac OS X\nLi
+nux    Linux of some sort\n\n";
do {
chomp ($OS = <STDIN>);
print "\nIncorrect OS type. Try again.\n\n" unless $OS eq "Windows" or
+ $OS eq "Mac" or $OS eq "Linux";} until ($OS eq "Windows" or $OS eq "
+Mac" or $OS eq "Linux");
}

[download]
Your "use if" construct is intriguing... My script also uses different modules on different platforms. So far, I've been resorting to changing the code for each platform (I need to produce separate releases for other reasons anyway) becuse if ($^O =~ /mswin/i) {use module::name;}
didn't work. Maybe I'll try use if $OS eq 'Windows', module::name;

In reply to Re^2: Modules and crossplatform code by elef
in thread Modules and crossplatform code by Khariton

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