my $fil = 'file.txt';
open(DATA, "$fil") || die "Can not open: $!\n";
my @dat = (<DATA>);
close(DATA);
open(DATA, ">$fil") || die "cant write: $!\n";

                                     
foreach (@dat)
{
    if($_ =~ /number/)
	{
		s/number/NEWWORD/;
		print DATA $_;
	}
}
close(DATA);