diff --git a/Build.PL b/Build.PL
index 51f286ed3..c87d5b51f 100644
--- a/Build.PL
+++ b/Build.PL
@@ -35,7 +35,7 @@ if (defined $ENV{CPANM} && -x $ENV{CPANM}) {
     $cpanm = $ENV{CPANM};
 } elsif (-x (my $c = File::Spec->catfile($Config{installscript}, 'cpanm'))) {
     $cpanm = $c;
-} elsif ($^O = /^(?:darwin|linux)$/ && system(qw(which cpanm)) == 0) {
+} elsif ($^O =~ /^(?:darwin|linux)$/ && system(qw(which cpanm)) == 0) {
     $cpanm = 'cpanm';
 }
 die <<'EOF'