sub intermix
{ 
    return (
        substr( $a, 0, rindex($a, '.'))
        eq 
        substr( $b, 0, rindex($b, '.'))
        )
    ? 1 : 0;
}
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The use of ?: is not necessary:

sub intermix
{ 
    return (
        substr( $a, 0, rindex($a, '.'))
        eq 
        substr( $b, 0, rindex($b, '.'))
        );
#    ? 1 : 0;
}
[download]