open (FILEIN,"$filein") or die "Open $filein failed: $!";
while (<FILEIN>) {
    @data = split;
    push @$output, [@data];
}    
close FILEIN;

#...other code that sets up the Excel sheet...

my $Book = $Excel->Workbooks->Open("$excelfile");
my $Sheet = $Book->Worksheets("$sheetname");

# ...other code that determines
#       $begcol,$begrow,$endcol,endrow

$Sheet->Range("$begcol$begrow:$endcol$endrow")->{'Value'} = $output;