my $resultFromExternalSource;
if( $^O eq 'MSWin32' ) {
    $resultFromExternalSource = win32GetFromExternalSource( ... );
}
elsif( $^O eq 'linux' ) {
    $resultFromExternalSource = linuxGetFromExternalSource( ... );
}
elsif( $^O eq 'VMS' ) {
    $resultFromExternalSource = ...;
}
elsif ...