#!/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;