Here's a solution that allows you to change either the full (@files) or required (@need) lists of files. The order you supply the elements to either list doesn't matter: the code handles that for you. Also, filenames are likely to contain characters outside of the [A-Z] range (e.g. a.txt, b.pl, c.png, etc.): I've wrapped these in \Q...\E to avoid interference with the regex.

use strict;
use warnings;

my @files = qw{ABC DEF GHI JKL MNO PQR STU};
my @need = qw{MNO ABC GHI};
my $need_re = join q{\s.*?} => map { q{\b} . qq{\Q$_\E} . q{\b} } sort
+ @need;

if (join(q{ } => map { qq{\Q$_\E} } sort @files) =~ /$need_re/) { 
    print qq{OK\n};
}   
else {
    print qq{NOT OK\n};
}
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