# read and subst
my $fh = new FileHandle($file,"<");
my @data = ();
while (my $line = <$fh>) {
  $line =~ s/$expr1/$expr2/;
  push(@data,$line);
}
close($fh);
# write new data
$fh = new FileHandle($file,">");
foreach my $line (@data) {
  print $fh $line;
}
close($fh);