use strict;
use warnings;

my @files_to_check = ( 'abc.txt', 'xyz.txt', 'efg.txt' );      # list of files to check

my @directory_listing = $ftp->ls($dir);

my %files_on_FTP_server;

foreach my $file (@directory_listing)
{
    # extra processing here?
    $files_on_FTP_server{$file} = 1;
}

foreach my $file (@files_to_check)
{
    if ( ! $files_on_FTP_server{$file} ) { print "File $file is missing\n"; }
}