use strict ; # now and forever!


my $db = 'aaa.txt';
open DATA, "< $db" or die "did not open for reading: $!\n";
my @dat = <DATA> ; # don't need parentheses, context forced by @dat
close DATA;

open DATA, "> $db" or die "did not open for writing: $!\n";
foreach my $line (@dat) {
  # Indent, it's useful!
  $line =~ s/main/index/g;  
  $line =~ s/\t/ /g;  #This is not working 
  $line =~ s/^/\a/g;   #This is not working
  print DATA $line;
}
close DATA ;