...

# Comparing columns 3 and 5

my $cmp_col_1 = 3;
my $cmp_col_2 = 5;

foreach my $sheet (@{$excel -> {Worksheet}}) {
    ...

    foreach my $row ($sheet -> {MinRow} .. $sheet -> {MaxRow}) {
        # Here's where you'd compare
        #   $sheet -> {Cells} [$row] [$cmp_col_1]
        # with
        #   $sheet -> {Cells} [$row] [$cmp_col_2]
    }
}