subcall( \@{$M[0]} );

Which passes a reference to the first array in @M.

So it does, but if the array  @M is initialized per something like
    my @M = (\@a1, \@a2, \@a3);
then the elements of  @M are all array references to begin with, so why not just
    subcall($M[0]);
and avoid taking a reference to a de-referenced array reference?

(And see also perllol "Manipulating Arrays of Arrays in Perl".) (Update: And also perlreftut.)

In reply to Re^2: Arrays in arrays, how to access them by AnomalousMonk
in thread Arrays in arrays, how to access them by viored

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