Tons of confusion over there, eh? :-)

First of all: you are opening your database and slurping in at once in memory. It's ok if it's not too big, but in case it is and you have enough disk space I suggest you to read line by line and write to another file. If you want, you could end your script by overwriting the old file with the newly created one; check File::Copy and it's function move for details.

Second: You use double quotes around $db in your opens: that would be needed if you specify, for example, "> $db" to mean overwrite the file. In your situation double quotes are useless.

Next: your foreach cycle doesn't print on DATA, which would be then left empty!

And finally: you used a character class \s where it had no meaning in the first substitution, and you swapped the matching string with the substitute in the second.

A slightly revised (but untested!) version of your code could be:

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 ;
[download]

Ciao!
--bronto

# Another Perl edition of a song:
# The End, by The Beatles
END {
  $you->take($love) eq $you->made($love) ;
}