$db = 'aaa.txt';
open(DATA, "$db") or die "did not open: $!\n";
@dat = (<DATA>);
close(DATA);
open(DATA, "$db") or die "did not open part 2: $!\n";
foreach $line (@dat)
{
$line =~ s/main/index/g;  
$line =~ s/\t/\s/g;  #This is not working 
$line =~ s/\a/^/g;   #This is not working
print $line;
}
close(DATA);