"1. I just want to merge 4 different xls files into a single file with 4 sheets."

The good news here is that you have code which already works for one input file. The sensible thing to do here is make some small alterations to your code and change it into a subrotuine. Rather than hard code values such as Infile.xls pass them to your subroutine as an argument. See also, Tutorial -> Subroutines -> Recursion: The Towers of Hanoi problem for a nice explanation and short example.

"2. I want the same format which comes with input file to be available in the result file which I'm sending to my end user(like first files format should be available in Sheet1 and second files input should be available in Sheet2 and so on)."

IIRC these modules don't preserve all formatting, you'd need to check the documentation.

"3.I have to assign Names to all 4 Sheets."

Pass them as an additional argument to your subroutine, along with the file name.

"4. Sheet Name has Special Character(/)(ie. From/To) and I want to assign the same to the Sheet Name"

I don't believe Excel lets you use the / character in the sheet name.

use strict;
use Spreadsheet::ParseExcel;
use Spreadsheet::WriteExcel;

my $parser = Spreadsheet::ParseExcel->new(); #Creating Object to Read 
+Worksheet

my @files = qw/ Infile1.xls Infile2.xls Infile3.xls Infile4.xls /;

my $write_workbook = new Spreadsheet::WriteExcel('Resultfile.xls');

my $n = 1;
foreach my $file (@files) {
    my $parse_workbook = $parser->parse($file);

    my $write_worksheet = $write_workbook->add_worksheet("Sheet$n");

    if ( !defined $parse_workbook) { die $parser->error(), ".\n"; }

    for my $parse_worksheet ( $parse_workbook->worksheets() ) {
        my ( $row_min, $row_max ) = $parse_worksheet->row_range();
        my ( $col_min, $col_max ) = $parse_worksheet->col_range();

        for my $row ( $row_min .. $row_max ) {
            for my $col ( $col_min .. $col_max ) {
                my $cell = $parse_worksheet->get_cell( $row, $col );
                next unless $cell;
                my $cell_value = $cell->value();
                my $cell_format = $cell->get_format();
                $write_worksheet->write($row, $col, $cell_value);
            }
        }
    }
    $n++;
}
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