#!/usr/bin/perl -w 
use strict; 
use diagnostics; 
if (!open FILE, "+<file.txt") {    die "Error: $!\n"    } # open file for read/write 
#$^I = ".bak"; 
while (<FILE>) #while reading strings from file 
{ 
    s/^\t//g; # erase tabs at the beginning of file , s/^\s//g do the same 
    print FILE; # print write to STDOUT 
} 
close FILE;

#!/usr/bin/perl -w 
use strict; 
use diagnostics; 
$^I = ".bak"; # if I commented this string, print go to STDOUT and file don't changed. Why? 
while (<>) 
{ 
    s/^\t//g; 
    print; 
}