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BEGIN { if ($^O =~ /Win32/) { ... use Win32::SerialPort; .. rest of your logic ...
use if $^O =~ /Win32/, "Win32"; use if $^O =~ /Win32/, "Win32::TieRegistry"; use if $^O =~ /Win32/, "Win32::SerialPort"; use if $^O !~ /Win32/, "Device::SerialPort";