The book 'Perl Best Practices' (ISBN 0596001738) suggests still another style for choosing a single value:

use strict;
use warnings;
use English;

sub GetPathnameSeparator {
    $_ = $OSNAME;
    my $seperator = 
          # OS      # Seperator
          /MACOS/i ? ':'
        : /WIN/i   ? '\\'
        :            '/'
        ;
    return $seperator;
}
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