You can test for definedness.

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use Excel::Writer::XLSX;
use Spreadsheet::ParseXLSX;

{   my $workbook = 'Excel::Writer::XLSX'->new('m.xlsx');
    my $worksheet = $workbook->add_worksheet;
    my $f = $workbook->add_format;
    $worksheet->merge_range(2, 2, 3, 3, undef, $f);
    $worksheet->merge_range(4, 4, 5, 5, 'merged', $f);
    $workbook->close();
}

{   my $parser = 'Spreadsheet::ParseXLSX'->new;
    my $workbook = $parser->parse('m.xlsx');
    my $worksheet = ($workbook->worksheets)[0];
    for my $x (1 .. 5) {
        for my $y (1 .. 5) {
            my $cell = $worksheet->{Cells}[$x][$y];
            print "[$x,$y] ", defined $cell ? $cell->is_merged : '-';
        }
        print "\n";
    }
}
__END__
[1,1] -[1,2] -[1,3] -[1,4] -[1,5] -
[2,1] -[2,2] 1[2,3] 1[2,4] -[2,5] -
[3,1] -[3,2] 1[3,3] 1[3,4] -[3,5] -
[4,1] -[4,2] -[4,3] -[4,4] 1[4,5] 1
[5,1] -[5,2] -[5,3] -[5,4] 1[5,5] 1
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