There wasn't any relevant tests for this sub to extend. But a trivial fix seems to solve the problem for Spreadsheet::ParseXLSX v 0.27:

 # (This is in Spreadsheet/ParseXLSX.pm.)

 sub _dimensions {
     my $self = shift;
     my ($dim) = @_;
 
+    $dim  = "A1:$dim"
+        if $dim !~ /:/;
+
     my ($topleft, $bottomright) = split ':', $dim;
     $bottomright = $topleft unless defined $bottomright;
     ...
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