open FH, '+<afile';
$temp = '';
  while (<FH>) {
  s/2/two\nhi hello/;
  $temp .= $_;
}
seek(FH, 0, 0);
print FH $temp;
truncate(FH, tell(FH));
close(FH);

##</code><code>##


sub slurp {
    my  $file = shift;
    open my $fh, '<', $file or return undef;
    local $/ unless wantarray;
    return <$fh> if defined wantarray;
}
sub jot {
    my $notepad = shift;
    open my $fh, ">", $notepad or die $!;
    print $fh @_;
}
my $temp = slurp 'file.txt';
$temp =~ s/2/two\nhi hello/;
jot 'file.txt', $temp;