G'day bowei_99,

Here's a solution using Tie::File you may find suitable.

#!/usr/bin/env perl

use strict;
use warnings;

use Tie::File;

my $file = 'pm_file_insert_re.file';
my $re = qr{ \A echo .* install_list \z }x;
my $insert_line = 'echo ":app:new-name-here:" >> /mnt/install_list';

tie my @lines, 'Tie::File', $file or die "Can't tie $file: $!";

my $last_found = 0;
@lines = reverse map {
    /$re/ && ! $last_found++ ? ($insert_line, $_) : $_
} reverse @lines;

untie @lines;
[download]

Here's a run showing before and after file contents.

ken@ganymede: ~/tmp
$ cat pm_file_insert_re.file
early text
echo ":os:5.7:" >> /mnt/install_list
middle text
echo ":app:name-here:" >> /mnt/install_list
later text
ken@ganymede: ~/tmp
$ pm_file_insert_re.pl
ken@ganymede: ~/tmp
$ cat pm_file_insert_re.file
early text
echo ":os:5.7:" >> /mnt/install_list
middle text
echo ":app:name-here:" >> /mnt/install_list
echo ":app:new-name-here:" >> /mnt/install_list
later text
ken@ganymede: ~/tmp
$
[download]