If the file you want to edit is very large (too large to suck the file comfortable into memory), and for some reason you don't want (or cannot) use a temporary file, you may want to do something like:

use Fcntl ':seek';

my $file_name = "...";

my $buffer = <<'EOT';
echo hello
echo world
echo I'm done.
EOT

open my $fh, "+<", $file_name or die $!;
my $r = 4096;
while ($r = sysread $fh, $buffer, $r, length $buffer) {
    seek $fh, -$r, SEEK_CUR or die $!;
    syswrite $fh, substr $buffer, 0, $r, "" or die $!;
}
die $! unless defined $r;
syswrite $fh, $buffer or die $!;
close $fh or die $!;
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The code above uses a fixed amount of memory, independent of the size of the file. You may want to do add a loop around syswrite if you think it may not write the entire chunk at once.