Well ... not quite. The require will die ... so you need eval. Also, perl treats barewords special here - if you use a string, you'll need it to be 'Win32/Service.pm'. Stick to the barewords when possible ;-)

$runningOn = 'unix';
eval {
  require Win32::Service;
  $runningOn = 'windows';
};
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Of course, we assume that Win32::Service never fails to load, but that's ok, since we make that assumption about most modules.

No. perldoc -f require :

... In other words, if you try this:

         require Foo::Bar;    # a splendid bareword
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The require function will actually look for the "Foo/Bar.pm" file in the directories specified in the @INC array.

But if you try this:

         $class = 'Foo::Bar';
         require $class;      # $class is not a bareword
     #or
         require "Foo::Bar";  # not a bareword because of the ""
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The require function will look for the "Foo::Bar" file in the @INC array and will complain about not finding "Foo::Bar" there.

In this case you can do:

         eval "require $class";
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...

Personally, I've used UNIVERSAL::require, which allows me to do

  "Win32::Service"->require()
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but I don't think that UNIVERSAL::require is really fit for production use.

My mistake. I use this

my @prereqs = (
        "DBI",
        "MIME::Base64");

foreach (@prereqs)
{
        if (!eval("require \($_\)"))
        {
                print "Missing prerequisite - $_\n";
                exit 0;
        }
}
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Which of course doesn't have "" in the require.

VGhpcyBtZXNzYWdlIGludGVudGlvbmFsbHkgcG9pbnRsZXNz

Believe it or not, but I benchmarked this ... and there is a slight advantage to not recompiling strings via eval. Depending on how hungry you are for CPU cycles...

foreach my $p (@prereqs)
{
  (my $x = $p) =~ s[::][/]g;
  $x .= '.pm';
  eval {
    require $x;
  }
  if ($@) {
    print "Missing prerequisite - $p\n";
    exit 0;
  }
}
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A bit more code - but a bit faster, too. The tradeoff is up to you.