use strict;
my @files = glob "file*.txt";
my $file;
foreach $file(@files){
  if($file =~ /^file(\d{4})\.txt$/){
    my $num = $1;
    $num =~ s/^0*//;
    open(IN, $file)||die "cannot open $file for read:$!\n";
    open(OUT, ">tmp.txt")||die "cannot open tmp.txt for write:$!\n";
    while(<IN>){
      s/stuff into/stuff where thing="$num" into/i;
      print OUT;
    }
    close IN||die "cannot close $file:$!\n";
    close OUT||die "cannot close tmp.txt:$!\n";
    rename('tmp.txt', $file);
  }
}