Take a look at Using Win32::OLE and Excel - Tips and Tricks.

This code demo shows how to remove the borders around a cell. You could adapt this to scan down the columns and use some logic around previous cell,next cell to remove the borders you don't want.

#!perl
use strict;
use Win32::OLE::Const 'Microsoft Excel';
Win32::OLE->Option(Warn => 3);

my $ex = Win32::OLE->GetActiveObject('Excel.Application')
        || Win32::OLE->new('Excel.Application', 'Quit');
        
# change filename to suit
my $wb = $ex->Workbooks->Open('c:\\temp\\border.xls') ; 
my $ws = $wb->sheets(1);
my $cell = $ws->Cells(7,2); # b7
$cell->Borders(7)->{LineStyle}  =  xlNone; #left
$cell->Borders(8)->{LineStyle}  =  xlNone; #top
$cell->Borders(9)->{LineStyle}  =  xlNone; #bottom
$cell->Borders(10)->{LineStyle} =  xlNone; #right

# save and exit
$wb->SaveAs( 'c:\\temp\\changedborder.xls' );
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