Passes same tests as above, but with no capture or length call, might be slightly faster — but no benchmarking done. (I passed all arrays by reference in my testing.)

sub Prefix {

    my ($ar_strings,
        ) = @_;

    return '' unless @$ar_strings;

    my ($m, $n) = (sort @$ar_strings)[0,-1];

    ($m ^ $n) =~ m{ \A \x00* }xms;

    return substr $m, 0, $+[0];

    }
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