The question is, do you know how to print the matrix at all? I'm going to assume you can, and if so instead of:

    # pseudocode
    for row (0..rows) {
        for col (0..cols) {
              print array[row][col]
        }
    }
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You substitute a list for the colums and rows you want to iterate over:

    # pseudocode
    for row (@goodrows) {
        for col (@goodcols) {
              print array[row][col]
        }
    }
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As an aside, this is interesting...

@gm=map { [ @$_[@goodcols] ] } @matrix[@goodrows];
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Giving a matrix with only the good columns and rows in it. Hmm. Although I'd have preferred some kind of @matrix[@goodrows]->[@goodcols] syntax, but I'll have to wait for Perl 6 for that.