xlText is a CONSTANT (which has the value -4158 ) so you don't quote it as this makes it a string. You also need to disable DisplayAlerts or you will get prompted with the usual are you sure BS.

use Win32::OLE::Const;
use Win32::OLE::Const 'Microsoft Excel';

my $Excel = Win32::OLE->new("Excel.Application" , sub { $_[0]->Quit } 
+) 
    or die Win32::OLE::LastError;
$Excel->{Visible} = 1;
my $Book = $Excel->Workbooks->Open("c:\\test.xls") 
    or die Win32::OLE::LastError;
my $Sheet = $Book->Worksheets(1);
$Excel->{DisplayAlerts} = 0;  # avoid being prompted
$Sheet->SaveAs( { FileName => "c:\\test.txt" , FileFormat => xlText } 
+) 
    or die Win32::OLE::LastError;
$Book->Close();
$Excel->Quit;
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