my @mat1 = readmatrix ($first);
my ($r1, $c1) = ($rows, $cols);

...

sub readmatrix {
    my ($file) = @_;
    open(IN, "$file") || die "can't open file: $!\n";
    while (defined (my $line = <IN>)) {
        my @tmp = split(' ', $line);
        $cols = scalar @tmp;
        $rows++;
        push (@AoA, [@tmp]);
    }
    close IN;
}